2aca4e2c02
Before this commit, if the value didn't begin with a dollar sign,
_zsh_highlight_main_highlighter__try_expand_parameter() would return 1
by accident.¹ Tweak the input validation to make this behaviour
explicit. No functional change.
¹ Specifically, it would return 1 because ${parameter_name}'s value
would be the empty string and ${parameter_name_pattern} wouldn't match
that.
zsh-syntax-highlighting / highlighters
Navigate into the individual highlighters' documentation to see
what styles ($ZSH_HIGHLIGHT_STYLES keys) each highlighter defines.
Refer to the documentation on highlighters for further information.