fix: Allow let: directive on slot element for Svelte 4 compatibility

[dimfeld]

Resolves #10382. The compiler validation phase was not allowing let: directives on slot elements, which Svelte 4 does allow for slot forwarding.

Svelte 5 rewrite

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.

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• 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
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• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

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

[changeset-bot]

🦋 Changeset detected

Latest commit: d74f763

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

This PR includes changesets to release 1 package

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svelte	Patch

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[dummdidumm]

This doesn't fix the issue yet, there's a runtime error now. Will continue working on this in your branch.

[dummdidumm]

Thank you!