Hi! Was going to write this as a comment but I thought a functional PR would be a nicer way to make a proposal.

A cleaned up version of this could resolve #943 in a way that didn't involve either inflating microbundle's size or completely rewriting the whole library around SWC.

Basically, it looks for the @swc/core dependency, and if it's found, assumes that the user wants to use swc instead of babel or typescript. But @swc/core isn't a dependency of microbundle. Which means most users can just continue using microbundle as is and not be affected. People who want to try microbundle out with SWC just have to install @swc/core which has the nice properties of being natural and intuitive for people who do want to use SWC, and unusual for anyone else.

It does add rollup-plugin-swc3 as a proper dependency of microbundle, because that's just a small javascript module, and it seems like the sort of thing microbundle should save users from having to worry about (whether they should use rollup-plugin-swc, rollup-plugin-swc2 or rollup-plugin-swc3, and how to configure the chosen package).

I tried on a small typescript project with no dependencies, with an output of about 4kb. With swc it took about 6s, without it took 21s (and using --compress false, timings were 3s vs 19s , see point 4 below).

Some things that should be addressed before this PoC were to be shippable (I didn't attempt to do this because I am not very knowledgeable about microbundle, babel, or SWC):

1. there should be a CLI flag to let people who have @swc/core installed for unrelated reasons to opt out (e.g. --swc false)
2. it should be configured properly to output the right style in various configurations. In this PR I just hardcoded target: 'es2018' fairly arbitrarily. It should also respect any existing swc config detected, if present.
3. docs!
4. the rollup-plugin-swc3 minifier should be used instead of rollup-plugin-terser - probably a bunch of the perf benefits are being negated by not doing this
5. (maybe) rely on rollup-plugin-swc3 to emit typescript declaration files too. Might or might not be faster 🤷
6. prettier/code cleanup etc. I deliberately avoided formatting src/index.js to make the diff easy to ready, but probably some refactoring should be done
7. (maybe) make @swc/core a peerDependency with peerDependenciesMeta.optional = true

If this goes well and people start using it, after a few months of the community helping flush out any issues, maybe something like the solution suggested by @developit in #943 (comment) could be done with SWC. But the advantage of doing this way first is that we could likely do exactly the same thing with rollup-plugin-esbuild, and effectively support users with both. The community seems pretty evenly split between the two, and they're both an order of magnitude faster than typescript or babel.