feat(completion): add native zsh completion support

[stefanoamorelli]

The current bash completion script uses bash-specific features like _init_completion and mapfile which don't exist in zsh. This causes zsh users to have no working completion for dnf5 1.

Previously, dnf4 shipped separate completion scripts for bash and zsh. This change restores that approach by adding a native zsh completion that uses dnf5's built-in --complete mechanism.

The implementation is intentionally minimal, using zsh's compadd to pass completions directly from dnf5's output. I tested more elaborate approaches with description parsing, but the simpler version proved more reliable across different zsh configurations.

Fixes #1826

[pkratoch]

I don't normally use zsh and I don't know how the completion should be defined, but with trying out this patch, the completion adds \ character when completing commands and options and when the options are listed, there is also \ before every space and parenthesis, like this:

# dnf in
info\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \(Lists\ packages\ depending\ on\ the\ packages\'\ relation\ to\ the\ system\ with\ additional\ details\)
install\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \(Install\ software\)

# dnf install\  

That doesn't seem intentional.

Also, I can't reproduce the original bug anymore, possibly thanks to 66e2f4c , but maybe I am just missing something.