chore: simplify hydration

[Rich-Harris]

Following #10939, we can simplify the hydration algorithm quite a bit. I need to add some comments to the code, but the basic idea is 'just in time hydration' — we call child(...) or sibling(...) or whatever to get the anchor node for the (if/each/await/etc) block that we're about to create, and if the child/sibling is a <!--[--> hydration opening marker, we populate hydrate_nodes and return the corresponding closing marker. We therefore never need to cache stuff on $$fragment, and anchor is always truthy. We also don't need to call hydrate_block_anchor inside blocks.

Follow-ups:

• get rid of $.open(...) and $.open_frag(...), we don't need them — we can just call the template function directly (and rename $.close(...) to something like $.append(...), since there's no longer any symmetry)
• get rid of ssr:if:true and ssr:each_else. just put that information in the closing hydration marker
• make each blocks more efficient — we don't need to wrap every item in hydration markers
• extend the 'controlled' optimisation to non-each blocks
• more useful errors when hydration fails (not yet sure what this looks like)
• disallow user comments that look like hydration markers
• see if we can make the types easier to work with. we're constantly having to cast to Node or TemplateNode, it would be nice if that was unnecessary

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

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0a68356

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR