breaking: remove baseUrl fallback from generated tsconfig

[dummdidumm]

• don't generate baseUrl anymore and don't adjust paths anymore depending on whether or not the user has paths in their tsconfig.json
• more strict validation: warn on baseUrl/paths and suggest kit.alias instead

closes #11286

also added a changeset for svelte-migrate because none was generated yet.

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Latest commit: 55b541e

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This PR includes changesets to release 2 packages

Name	Type
@sveltejs/kit	Major
svelte-migrate	Minor

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