fix: don't migrate $: foo.bar = baz to use $derived()

[Ocean-OS]

This ensures that during migration, $deriveds are only applied to Identifiers, and not object properties.
Fixes #13723.

Svelte 5 rewrite

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[changeset-bot]

⚠️ No Changeset found

Latest commit: e831d52

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[dummdidumm]

Looks good! Could you add a test (within tests/migrate) and add a changeset (via npx changeset at the root of the repo then follow the prompts)?

[dummdidumm]

merging this into #13740 and gonna add a test / changeset there - thank you!