DotfileManager: add initial dotfile manager detection

[twpayne]

Fixes #1696.

[CarterLi]

Optionally, check version of these managers.

[twpayne]

Thanks for the speedy review! I'll have time to work on this again in a few days and will address all your comments.

[r1w1s1]

maybe its a way to add stow support?

Fastfetch could add a new module (e.g., dotfiles) that prints:

Dotfiles Manager: Stow (~/dotfiles)

And detects it using logic like:

Scan $HOME for symlinks.

If > N symlinks point into a common dir (~/dotfiles/*), assume Stow is used.

Optionally allow override via config.