fix: correctly validate undefined snippet params with default value

[paoloricciuti]

Closes #15749

Technically we could've avoid all the fallback_idx conundrum and just allow undefined too but this should be more correct most of the times and it's not really a problem since it's mostly in dev. I also thought of using a Set instead of an array but it's not really worth it since there will never be a lot of params anyway.

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🦋 Changeset detected

Latest commit: da36864

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

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Playground

pnpm add https://pkg.pr.new/svelte@15750

[Rich-Harris]

we can simplify this by just using arguments — will push a change

[paoloricciuti]

Uh that's frickin smart