fix: tighten up $ prefix validation

[dummdidumm]

Our current validation was both too lax and too strict:

• too strict, because you can define a $ variable in Svelte 4 if it's not at the top level
• too strict, because you can define $-prefixed function parameters, but not give the parameter the name $
• too lax, because you can define a $-prefixed variable if you're at least one level deep into a function. In runes mode, this should be an error

This PR aligns the behavior, ensures this isn't a breaking change in legacy anymore, and makes the validation in runes mode more strict

Found out through this commit from another repo

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[changeset-bot]

🦋 Changeset detected

Latest commit: 675844e

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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