fix: improve error message for TS-like snippet annotations without lang="ts"

[samuelkal]

This PR improves the error message when a snippet in Svelte contains TypeScript-style type annotations
(e.g., {#snippet bar(text: string)}) but the <script> tag does not have lang="ts".

Without this fix, users get a generic "Unexpected token" error, which is confusing.
Now, the compiler shows a more helpful message:
"Unexpected token ':' – did you forget to add lang='ts'?"

References issue: #16835

Tests have been run with pnpm test and linting with pnpm lint. Only the error message output has been adjusted.

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🦋 Changeset detected

Latest commit: 0b62c63

The changes in this PR will be included in the next version bump.

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[dummdidumm]

Thank you for starting a PR for this. I think the given solution won't work though.

Consider this snippet:

{#snippet aa({x: y})}
	{y}
{/snippet}

{@render aa({x: 'hi'})}

This matches your pattern and would fail wrongfully. I think it would be better to check once the error is thrown instead, look for the : or similar triggers and then add the "did you mean to add ... lang=ts" if we know the script tag doesn't already have it.

[github-actions]

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