shell completion for mangohud & mangohudctl

[realchonk]

This is a small PR for shell completions (currently only bash). It adds a new meson build option -Dbashcompletiondir=. Unfortunately, I've got neither experience with zsh nor with fish completions, but it should be pretty simple to add. Just look into data/meson.build and data/shell-completion/bash/meson.build for reference.

[evelikov]

Assuming this gets anywhere, I can zsh completions o/

[realchonk]

Assuming this gets anywhere, I can zsh completions o/

Yeah, that would be good.