feat: add zsh-completion

[AtifChy]

#1197

This pull request introduces completion for zsh. I've customized the python script originally used to generate fish shell completion to generate zsh completion.

[CarterLi]

How can I test it?

[AtifChy]

How can I test it?

fpath=(repos/fastfetch/completions $fpath)

you may need to also run autoload -Uz compinit && compinit

then run fastfetch <TAB>

[CarterLi]

• _fastfetch should be renamed to fastfetch.zsh
• This file should be installed by cmake. See https://github.com/fastfetch-cli/fastfetch/blob/dev/CMakeLists.txt#L1340-L1344

[AtifChy]

• _fastfetch should be renamed to fastfetch.zsh

but this is how zsh completion files are named

[CarterLi]

The file can be renamed when installing

[AtifChy]

• This file should be installed by cmake. See https://github.com/fastfetch-cli/fastfetch/blob/dev/CMakeLists.txt#L1340-L1344

done

[CarterLi]

Thanks