fix: handle optional and rest routes for ISR correctly

[dummdidumm]

When turning on ISR in adapter-vercel for a route ending in an optional or rest parameter, and that parameter not being set, the regex would yield wrong results. Consider a route like foo/[...rest] and the URL is just /foo, then the route config for Vercel would turn that into a ISR-request like foo/[..rest]?pathname/%2Ffoo%2F - note the trailing slash, which comes from the regex looking like foo/$1/$2. This trailing slash which wasn't there before can subsequently lead to the SvelteKit runtime issuing a trailing slash redirect (it thinks we're on /foo/ and issues a redirect to /foo), which is wrong an will lead to infinite redirects in the browser. The fix is to account for optional and rest parameters when constructing the regex for the Vercel route config.

Fixes #10836
Fixes #10155

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[benmccann]

This probably fixes #10836 at least and maybe a couple of others.