chore: refactor rendering and transitions

[Rich-Harris]

Track progress here

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This is very WIP, and lots of tests are failing.

Right now, there's a lot of complexity around rendering and transitions that I'm very confident is unnecessary. For each thing like {#if ...}, {#each ...} etc we have a corresponding block type, each with a different shape, and when we interact with the tree in some way we need to lots of type === IF_BLOCK checks and suchlike. Similarly, we have different kinds of transitions for different kinds of blocks.

We don't need any of this — we just need the effect tree, and a single type of block:

<!-- this is a render effect -->
{#if condition}
  <!-- this is a block -->
{:else}
  <!-- this is also a block -->
{/if}

Here, a 'block' is technically a 'managed render effect' — the terminology could probably be improved somewhat, since the consequent/alternate functions don't re-run autonomously. They're managed, but they're not effects. Nonetheless, they are part of the effect tree insofar as they contain effects and are contained by effects, so for now we'll gloss over it.

Transitions belong to their immediate block (i.e. to the consequent or the alternate rather than the {#if ...}, or the pending block rather than the {#await ...}). This PR introduces the idea of pausing and resuming these blocks; the logic is shared between if and each and await and key and everything else.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

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

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[Rich-Harris]

I'm declaring bankruptcy on this branch, but will keep it around as a convenient reference. The ideas are sound, but it became too unwieldy to land. Going to try and reimplement it more incrementally.