fix: recognize all custom element prop definitions

[dummdidumm]

We didn't account for the $props rune being writtin in a way that makes some props unknown, and they would only be visible through the customElement.props definition. This changes the iteration to account for that and also adds a note to the documentation that you need to list out the properties explicitly.

fixes #13785

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[changeset-bot]

🦋 Changeset detected

Latest commit: ab873d1

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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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14084

commit: ab873d1