Easier selection of known release channels

[hasufell]

1. ask during bootstrap script which release channels should be activated
2. maybe have aliases for known release channels, so that the user can do ghcup config add-release-channel prereleases
   • where should the mapping exist? In ghcup code, another yaml or in one of the "base" yamls?
3. Support selecting/managing release channels via the TUI

@lsmor

[lsmor]

About the first two points.... I got no idea. About this:

3. Support selecting/managing release channels via the TUI

Surely we can a new button in the contex menu, with "Add release channel" and something like this

|--------------- Add New Channel -----------------|
|                                                 |
| From known Channels: ◦ nightlies                |
|                      ◉ prereleases              |
|                      ◦ latest                   |
|                                                 |
| From URL:  [ Enter your URL here           ]    |
|---------------- Add New Channel ----------------|