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.

CHANGELOG.md

@@ -43,6 +43,7 @@
 - Fixed bug caused by using `--plain` and `--terminal-width=N` flags simultaneously, see #3529 (@H4k1l)
 - Fixed syntax tests path, see #3610 (@foxfromworld)
 - Fix zsh tab completion word-splitting language names containing spaces (e.g. `HTML (Jinja2)`, `Apache Conf`), see #3693 (@YoshKoz)
+- Fix zsh tab completion offering invalid `-l` arguments (file globs, paths, hidden filenames) sourced from the second column of `--list-languages`. Closes #3735, see #PR_NUMBER (@truffle-dev)
 
 ## Other
 - Use git version of cross. See #3533 (@OctopusET)