This updates the docs and the driver, in a manner backwards compatible with
existing highlighters. (None of the highlighters are touched by this change,
yet tests continue to pass.)
Part of issue #329.
print -r -- >&2"zsh-syntax-highlighting: '${highlighter}' highlighter should define both required functions '_zsh_highlight_${highlighter}_highlighter' and '_zsh_highlight_${highlighter}_highlighter_predicate' in '${highlighter_dir}/${highlighter}-highlighter.zsh'."
print -r -- >&2"zsh-syntax-highlighting: '${highlighter}' highlighter should define both required functions '_zsh_highlight_highlighter_${highlighter}_paint' and '_zsh_highlight_highlighter_${highlighter}_predicate' in '${highlighter_dir}/${highlighter}-highlighter.zsh'."