Move cli extension to `blitz-next`

- [ ] `blitz next CMD` will be special cased, such that all nextjs commands like `blitz dev`, `blitz lint`, etc will run `blitz next dev`, etc.
- [ ] `blitz dev`, `blitz build`, and `blitz start` all need to support custom server (see https://github.com/blitz-js/blitz/blob/canary/packages/server/src/dev.ts)
- [ ] `blitz codegen` for Routes manifest
- [ ] `blitz dev` should continue to automatically find next port

Start by locating the current CLI extension and read packages/server/src/dev.ts for the custom-server behavior. Trace how blitz dev, build, start, codegen, and next commands are currently handled, including automatic Next port selection. Done means the listed commands work through blitz-next with custom servers, Routes manifest codegen, and preserved port discovery.

Written by the indexing model from the issue text.