Replace prompt-based RepoInit with full fledged TUI

[mattprecious]

The current flow for initializing stacker in a repo is slightly cumbersome with the 'trailing trunk' prompt that 99% of people will not use. This will only get worse as more configuration options are added. This should be replaced with something that resembles a traditional settings UI:

┌────────────────┬──────┐
│ trunk          │ main │
├────────────────┼──────┤
│ trailing trunk │    - │
└────────────────┴──────┘

   Cancel    |    OK

If you're happy with the defaults we've selected, then you just press Enter. Otherwise, you can arrow/mouse around and change settings as needed.