[FEATURE] Add shell completions macros

[Tbusk]

Proposal

Currently you need to use the %install_file macro paired with figuring out where to put the files for each completion, which is hard to remember.

The current way looks like this:

    %install_file extra/completions/alacritty.bash $installdir/usr/share/bash-completion/completions/alacritty
    %install_file extra/completions/_alacritty $installdir/usr/share/zsh/site-functions/_alacritty
    %install_file extra/completions/alacritty.fish $installdir/usr/share/fish/vendor_completions.d/alacritty.fish

It could look like this

    %install_bash_comp extra/completions/alacritty.bash
    %install_zsh_comp extra/completions/_alacritty
    %install_fish_comp extra/completions/alacritty.fish

Use cases

Installing shell completions easily and removing room for error.

[EbonJaeger]

I would rather do what AerynOS does and add location variables/macros, like:

%install_file foo.bash ${installdir}/%bashcompletiondir%/foo

I've avoided adding the various location macros so far because some of them depend on YPKG internal code, and I haven't yet had time to finish my rewrite project of all of that.