Router should support custom 404 in dev

[wmertens]

What is it about?

custom 404.tsx should be easy to develop

What's the motivation for this proposal?

Router only has custom 404 possible after SSG, because they get hardcoded into the build.

Proposed Solution / Feature

Instead, there should supoort for 404 in the route resolution, and running the router should include rendering the 404, except if a flag fallthrough: true is passed to the router middleware.

This will also allow custom 404 support when SSG is turned off, then the component will render like regular pages.

This will also tie in nicely with #295.

Links / References

No response