Ability to define "sub-apps", e.g. src/(app), e.g. src/(auth) or src/(stripe) with lib, components, routes etc.

[petr-rozum-ai]

From code organization perspective, it would be great if one could structure codebase like so:

src/
  (app)/
    lib/
    components/
    routes/
  (auth)/
    lib/
    components/
    routes/
      sign-in/
      sign-out/
  (stripe)/
    lib/
    components/
    routes/
      pricing/
      billing/
      webhook/

This would make it easier to create reusable self-contained pieces of functionality. As it is right now, the lib, components etc. of each of these is spread out.

Open question: Where should the sub-app routes be mounted? It could be either the root, or a provided name. For example, if using (...) it could be the root, otherwise the directory name.