[DX] show the currently active branch

[kiwicopple]

Feature request

Is your feature request related to a problem? Please describe.

Below these, it would be good to show the active branch:

Describe the solution you'd like

You're currently on the `main` database branch.
Run 
  `supabase db branch create` to create a new branch
  `supabase db branch switch` to switch to an existing branch
  `supabase db branch list` to see all your branches