fix(zsh): use newline-splitting for language completions to fix word-splitting on names with spaces

Language names like "HTML (Jinja2)" and "Apache Conf" contain spaces, which
caused $() command substitution to word-split them into garbage tokens. Use
${(f)"$(...)"}  (split on newlines only) to match the pattern already used
for theme completions on line 100.

Fixes #2897