fix: only offer language names in zsh tab completion for `-l`

The previous awk script in `bat.zsh.in` split each line of
`bat --list-languages` on `:` or `,` and emitted every field as a
completion candidate, including the second column. That column lists
file matchers, which can be plain extensions (`rs`), globs (`*.rs`),
absolute paths (`/etc/profile`), or filenames (`Containerfile`). None
of those parse as `-l` arguments, so completing them produces
`unknown syntax` errors.

Switch to splitting on `:` only and emit the language name as the
completion value with the file-matcher list as its description, which
matches the bash completion's behavior.

Closes #3735.