The way we handle branches is kinda broken in an async world. We assume — quite reasonably unless you're in an async world — that it's safe to simply bail out of a block if the expression is unchanged from before. When it's time to commit a block, we commit whichever branch corresponds to the most recent invocation, which isn't correct. And it's only possible to have one offscreen branch per block. And so on. I'm pretty sure there's some memory leaks in there as well.

This PR adds a new BranchManager class, which each block type can use to control which branches are created when. It ensures that branches are correctly linked to batches, and that they're destroyed at the appropriate time. It handles all the complexities around transitions and as-yet-unresolved batches.

Currently it's used for #if, #await, #key, @render, <svelte:component> and <svelte:element>. It might make sense to use it for <svelte:boundary> as well. It does not currently power #each blocks, which are unique in that their branches aren't mutually exclusive.

I'm hoping that this will enable more work towards #16971 and related issues.

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