Add fish shell completions support

[natsukagami]

What's this?

Fish is a modern, popular non-UNIX compatible shell with lots of builtin features.
By supporting Fish we also expand support to Nu, another modern shell with Fish completions compatibility.

The Fish shell completions format is extremely simple (full documentation here).
In general, program-generated fish completion list is just a list of lines, where each item can be followed by an optional description, separated by a tab \t character.

While in general Fish prefers a mostly-static completion list (see the documentation for more details), I think for simplicity's sake it's easier to just keep everything dynamic for now; it also has the upside of supporting non-CaseApp framework conformant projects like scala-cli.

What's included?

• A fish shell implementation of the completion script.
  • Fish does not require modification of its rc file to load completions (yay modernity!). Instead we only need to install the completions file to the correct place.
    The default install location is $XDG_CONFIG_HOME/fish/completions, but this can be changed with the --output/-o flag.
  • It is also possible to ship this completion file statically when bundling apps, so a completions install command might not even be needed.
• Re-factoring out completionsInstall and completionsUninstall methods to the object.
  • The main goal is to decouple the two commands from the framework hierarchy. This would help scala-cli for example to just re-use case-app's implementation and not have to duplicate all the logic.
  • I do agree this introduces a few new parameters that looks a bit ugly, feel free to change this to whatever suited.

[Gedochao]

@alexarchambault would you have the time to take a look at this?

[natsukagami]

I had to reset mima, since I introduced a new field in the CompletionsUninstallOptions case class. @alexarchambault

[alexarchambault]

Thanks @natsukagami! (and @Gedochao for the heads'up)