fix: correctly match route groups preceding optional parameters

[teemingc]

fixes #13095

This PR changes the RegExp to ensure that routes such as /[[optional]]/(group) are treated the same as [[optional]]. Previously, /[[optional]]/(group) was being treated the same as / which is incorrect since it has an optional segment. It wasn't recognised as /[[optional]] because the RegExp didn't consider the route group preceding it (it only looked for the end of the string).

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13099-svelte.vercel.app/

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