fix(migration): named slots with reserved keywords

[TorstenDittmann]

Fixes named slots with a reserved keyword not working anymore after migration. Re-uses the @migration-task logic for invalid identifiers.

Fixes #14277

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[changeset-bot]

🦋 Changeset detected

Latest commit: c9d218b

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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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14278-svelte.vercel.app/

this is an automated message

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14278

[dummdidumm]

Thank you! Could you add a changeset? Then we're good to go

[TorstenDittmann]

Thank you! Could you add a changeset? Then we're good to go

done :)