fix: allow for duplicate var declarations

[paoloricciuti]

Closes #15375

There was a comment above the line as you can see...while i agree with the comment i think we should try to stick with JS when possible. I tried to also include functions in this but there's an upstream bug in acorn that flag double function declarations as a js parse error.

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Preview: https://svelte-dev-git-preview-svelte-15382-svelte.vercel.app/

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[trueadm]

Agreed, we should be sticking to JS semantics where possible so we teach people the caveats of the language too.