feat: add applyReroute helper for catch-all serverless functions

[teemingc]

alternative to #16590

fixes #11879

This PR makes the assumption that a catch-all serverless function is deployed with no routes in its manifest (the alternative was an explicit flag). It then exports a helper which should be called in such a serverless function after server.respond so that it can invoke the correct handler instead of simply returning a 404 since it has no route information in its SSR manifest.

The advantage of this approach compared to deploying a middleware that only runs reroute is that it's simpler implementation-wise and it allows the user's handleFetch, init hooks, etc. to run correctly without any additional effort from us.

The disadvantages are that:

• reroute will run twice: once in the catch-all function then again in the correct function. We could choose to skip the second reroute based on some header value or context (if any) that can be passed between serverless functions. Otherwise, I chose to omit this so that the reroute hook can't be skipped from outside influence (someone setting the header themselves without running reroute first)
• Slightly bigger serverless function bundle than a middleware which just runs reroute. Not that big of an issue I think...

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