feature: `ghcup list` should be able to filter by tool

[ulysses4ever]

❯ ghcup list ghc                                                                                                              
Invalid argument `ghc'

I'd like the above to produce a list of GHC versions available.

[ulysses4ever]

Oh, after trying ghc list --help, I found out that there's -t that filters by tool. Why isn't it the normal argument but a flag? I think ghcup list ghc should work...

[hasufell]

I think the idea was to be able to specify -t multiple times. A flag is more proper for that.

[ulysses4ever]

Mm… I’d guess one tool is a by-far more common use case but I don’t have the data. Would ghcup list ghc stack be bad? Not sure why… cabal build expects 0 or more targets and that seems to work fine.

[hasufell]

It's inconsistent with the rest of the cli interface.

ghcup install ghc stack doesn't work and can't work properly.

Arguments, in ghcup interface, open a new context (called subcommand in optparse-applicative).