fix rest param type generation

[mrkishi]

Fixes #4327.

For type generation, we manually extract the param names from the path, but we missed the existence of the spread operator.

We could get the canonical list of params from the manifest data for routes, but we don't have them for components (layouts and errors). Perhaps the manifest data could be refactored a bit so this would be available, but I have no thoughtful opinions for now—this just removes the spread operator from the names.

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