chore: centralise branch management

[Rich-Harris]

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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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: f275aa3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16977-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16977

[Rich-Harris]

confession, I don't really understand why this change was necessary. but it was

[dummdidumm]

Very nice how this deduplicates the logic!