fix: Add check for builtin custom elements in set_custom_element_data

[edoardocavazza]

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Fixes #16591

This PR introduces a check for builtin custom elements (is attribute) inside the set_custom_element_data helper in order to correctly set properties that have a setter.

I would like to add tests but I am not familiar with the codebase and wasn't able to find the correct path.

[changeset-bot]

🦋 Changeset detected

Latest commit: 2117a15

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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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16592

[dummdidumm]

Thank you, that makes sense. For a test, you can e.g. enhance the one you created back in #15086: Add a <div is="value-element" value="test"></div> to the bottom and check that it outputs the correct thing.

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16592-svelte.vercel.app/

[edoardocavazza]

@dummdidumm Added the test case!

[dummdidumm]

thank you!