Fixes:
```
luajit: ...and-t/bin/benchmarks/../../lua/wincent/commandt/init.lua:199: attempt to call field 'nvim_buf_is_valid' (a nil value)
```

Fixes:
```
luajit: ...and-t/bin/benchmarks/../../lua/wincent/commandt/init.lua:244: attempt to index field 'scanners' (a nil value)
```

Vendoring from:
- https://github.com/microsoft/mimalloc
and specifically:
- https://github.com/microsoft/mimalloc/releases/tag/v2.0.6
I added a script to pull down the release archive and dump it into a
directory, because I don't want to use a submodule for this (people
installing a Vim plugin from a Git repo shouldn't have to know/worry
about whether it needs or uses submodules). Space on disk for this set
of files (some of which are obviously redundant in our context) is:
 du -sh lua/wincent/commandt/lib/vendor/github/microsoft
 4.8M lua/wincent/commandt/lib/vendor/github/microsoft
As it is not clear whether this is going to be a great idea or not, it
only takes effect if you call `make` with `USE_MIMALLOC` set. You can
verify that it actually _is_ overriding the standard `malloc()` etc
calls by running a command with `MIMALLOC_VERBOSE`, which will cause it
to print some extra info out:
 env MIMALLOC_VERBOSE=1 TIMES=1 bin/benchmarks/scanner.lua
Impact (unfortunately, a bit inconclusive) on scanner and matcher
benchmarks follows. Note that numbers shouldn't be compared across
machines because they were produced at different times (for example, the
M3 numbers are from a different version of the OS, and the branch was
rebased, compared with the other machines).
On mid-2015 MacBook Pro
=======================
These numbers are all over the map due to thermal throttling.
 best avg sd +/- p (best) (avg) (sd) +/- p
 buffer 0.04094 0.04178 0.00278 [-0.6%] (0.04100) (0.04186) (0.00287) [-0.6%]
 file 0.30707 0.31436 0.02486 [-1.0%] 0.05 (0.30735) (0.31473) (0.02499) [-1.0%] 0.05
 find 0.05827 0.06678 0.01162 [+1.5%] 0.05 (0.92013) (0.93752) (0.04453) [-1.0%] 0.025
 git 0.05163 0.06000 0.01115 [+3.3%] 0.0005 (1.00993) (1.02469) (0.04072) [-0.7%] 0.025
 rg 0.06419 0.07229 0.01203 [+3.8%] 0.005 (1.61018) (1.66326) (0.08803) [+0.3%]
 watchman 0.01095 0.01121 0.00068 [+0.2%] (1.16830) (1.17605) (0.01835) [+0.6%] 0.005
 total 0.54387 0.56643 0.04391 [+0.4%] (5.09873) (5.15811) (0.15328) [-0.1%]
 best avg sd +/- p (best) (avg) (sd) +/- p
 pathological 0.44648 0.48275 0.19826 [-10.0%] 0.01 (0.44705) (0.48350) (0.19793) [-10.0%] 0.01
 command-t 0.41205 0.44292 0.21658 [+3.8%] 0.005 (0.41255) (0.44364) (0.21681) [+3.8%] 0.005
 chromium (subset) 2.75724 2.99017 0.47925 [-1.3%] (0.51232) (0.55960) (0.17228) [-1.5%]
 chromium (whole) 3.18933 3.63241 0.64392 [-0.7%] (0.41821) (0.49571) (0.14853) [-0.3%] 0.05
 big (400k) 4.90155 5.51271 1.20748 [-1.0%] (0.65297) (0.74723) (0.23045) [-4.5%] 0.05
 total 11.74815 13.06097 2.16866 [-1.2%] (2.47007) (2.72968) (0.54795) [-2.8%] 0.025
M1 MacBook Pro
==============
 best avg sd +/- p (best) (avg) (sd) +/- p
 buffer 0.04407 0.05368 0.01123 [-1.4%] 0.025 (0.04433) (0.05413) (0.01150) [-1.6%] 0.025
 file 0.20902 0.21428 0.01060 [+1.0%] 0.01 (0.20902) (0.21511) (0.01219) [+1.1%] 0.005
 find 0.02687 0.03006 0.01015 [+3.9%] 0.05 (0.63141) (0.64156) (0.03483) [+0.7%] 0.05
 git 0.02693 0.02995 0.00980 [+2.2%] (0.71734) (0.72825) (0.04266) [-0.4%]
 rg 0.02916 0.03318 0.01136 [+2.9%] (0.90193) (0.91710) (0.07157) [+1.4%] 0.005
 watchman 0.01100 0.01156 0.00165 [-0.7%] (1.18802) (1.21274) (0.13422) [+1.5%] 0.005
 total 0.36119 0.37272 0.03632 [+1.1%] (3.71713) (3.76889) (0.18577) [+0.9%] 0.005
 best avg sd +/- p (best) (avg) (sd) +/- p
 pathological 0.28526 0.29636 0.08356 [-4.0%] 0.025 (0.28527) (0.29647) (0.08343) [-4.0%] 0.025
 command-t 0.23759 0.24616 0.07356 [+1.6%] (0.23760) (0.24618) (0.07354) [+1.6%]
 chromium (subset) 1.56761 1.58469 0.03655 [-0.3%] (0.41376) (0.42040) (0.02032) [-0.4%]
 chromium (whole) 1.87180 1.88726 0.06174 [-0.4%] 0.025 (0.31695) (0.32809) (0.03497) [+0.4%]
 big (400k) 2.90455 2.92204 0.07185 [-0.2%] (0.48384) (0.50533) (0.07608) [-0.0%]
 total 6.88851 6.93650 0.15002 [-0.4%] 0.025 (1.74550) (1.79647) (0.14517) [-0.5%]
M3 MacBook Pro
==============
 best avg sd +/- p (best) (avg) (sd) +/- p
 buffer 0.01255 0.01400 0.00409 [+2.0%] (0.01260) (0.01447) (0.00635) [-3.3%]
 file 0.14749 0.15026 0.00629 [+38.1%] 0.0005 (0.14843) (0.15115) (0.00626) [+37.9%] 0.0005
 find 0.20783 0.27306 0.12796 [+15.8%] 0.0005 (1.13360) (1.38588) (0.55490) [+15.3%] 0.0005
 git 0.21748 0.25155 0.10398 [+13.0%] 0.0005 (1.17693) (1.40937) (0.54965) [+9.1%] 0.0005
 rg 0.20640 0.26983 0.12977 [+12.2%] 0.0005 (1.55310) (1.78037) (0.55921) [+6.9%] 0.0005
 watchman 0.01813 0.01980 0.00287 [+6.1%] 0.0005 (1.19740) (1.21007) (0.02198) [-0.2%]
 total 0.81542 0.97850 0.33560 [+17.1%] 0.0005 (5.23262) (5.95132) (1.66475) [+8.7%] 0.0005
 best avg sd +/- p (best) (avg) (sd) +/- p
 pathological 0.21079 0.22604 0.10943 [+4.8%] 0.025 (0.21107) (0.22640) (0.10972) [+4.7%] 0.025
 command-t 0.16694 0.17164 0.04923 [-0.6%] (0.16716) (0.17228) (0.05253) [-0.5%]
 chromium (subset) 1.35310 1.36239 0.02010 [+0.1%] (0.28797) (0.29255) (0.01108) [+0.3%]
 chromium (whole) 1.11148 1.11599 0.01258 [+0.3%] 0.01 (0.12167) (0.12478) (0.00828) [-0.2%]
 big (400k) 1.67454 1.68249 0.05630 [+0.6%] 0.0005 (0.18195) (0.18487) (0.00876) [+0.0%]
 total 4.52863 4.55855 0.15573 [+0.5%] 0.01 (0.97644) (1.00087) (0.12712) [+1.0%]
Ryzen 5950X Arch Linux
======================
 best avg sd +/- p (best) (avg) (sd) +/- p
 buffer 0.02465 0.02544 0.01098 [-0.4%] (0.02467) (0.02546) (0.01099) [-0.5%]
 file 0.09906 0.09948 0.00124 [-0.1%] (0.09943) (0.09995) (0.00130) [-0.2%]
 find 0.01852 0.01885 0.00084 [+0.5%] (0.25137) (0.25430) (0.00762) [+0.1%]
 git 0.01718 0.01811 0.00210 [+0.6%] (0.22095) (0.22468) (0.01156) [-0.6%]
 rg 0.01748 0.01792 0.00105 [+0.5%] (0.60575) (0.61077) (0.01562) [-0.1%]
 watchman 0.00178 0.00186 0.00033 [-5.6%] (0.02282) (0.02717) (0.02826) [-11.5%]
 total 0.17975 0.18165 0.01018 [-0.0%] (1.23025) (1.24233) (0.04061) [-0.4%] 0.05
 best avg sd +/- p (best) (avg) (sd) +/- p
 pathological 0.26186 0.27703 0.10940 [-4.4%] 0.0005 (0.26196) (0.27715) (0.10946) [-4.4%] 0.0005
 command-t 0.19271 0.20058 0.05044 [-3.0%] 0.0005 (0.19279) (0.20065) (0.05047) [-3.0%] 0.0005
 chromium (subset) 1.83627 1.89158 0.25631 [-3.8%] 0.01 (0.45977) (0.49985) (0.21028) [-15.7%] 0.005
 chromium (whole) 1.36877 1.38916 0.06031 [+2.6%] 0.0005 (0.12129) (0.12530) (0.01659) [-0.4%]
 big (400k) 2.39053 2.43636 0.11813 [+1.8%] 0.0005 (0.19600) (0.20396) (0.02644) [-0.1%]
 total 6.09256 6.19472 0.33431 [-0.2%] (1.24139) (1.30690) (0.25114) [-7.5%] 0.005

The .prettierignore change is because there are a couple of things in
the Markdown files that Prettier doesn't like.
The clang-format thing comes from a tip here:
- https://stackoverflow.com/a/57272592/2103996
Should prevent CI failures like this one:
- https://github.com/wincent/command-t/actions/runs/2979207632 

Wasn't needed on clang, but is needed with gcc:
 /usr/bin/ld: mimalloc-override.o: relocation R_X86_64_TPOFF32
 against `recurse' can not be used when making a shared object;
 recompile with -fPIC

I can't see a changelog or release notes in the repo, so here is the
diff:
- microsoft/mimalloc@v2.0.6...v2.1.7