Svelte 5 rewrite

Closes #13464

I've added a migration task comment at the beginning and since afterUpdate and beforeUpdate was not throwing i added the throw for them too. Since i was at it i thought that sometimes we just left imports lingering there and it would be a shame not migrating an otherwise migratable file just for this so i added a bit of "treeshake" logic to remove the import if it's not used.

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

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