This introduces a new variable, $fish_color_option, that can be used
to highlight options differently.
Options are tokens starting with `-`, but only up to (and including!)
the first `--`.
Fixes#8292.
This introduces a new variable $fish_color_keyword that will be used
to highlight keywords. If it's not defined, we fall back on
$fish_color_command as before.
An issue here is that most of our keywords have this weird duality of
also being builtins *if* executed without an argument or with
`--help`.
This means that e.g.
if
is highlighted as a command until you start typing
if t
and then it turns keyword.