patch 9.1.1138: cmdline completion for :hi is too simplistic

Problem:  Existing cmdline completion for :highlight was barebone and
          only completed the highlight group names.

Solution: Implement full completion for the highlight group arguments
          such as guifg and cterm. If the user tries to complete
          immediately after the '=' (e.g. `hi Normal guifg=<Tab>`), the
          completion will fill in the existing value, similar to how
          cmdline completion for options work (Yee Cheng Chin).

closes: #16712

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/highlight.pro b/src/proto/highlight.pro
index 33f90c6..5318258 100644
--- a/src/proto/highlight.pro
+++ b/src/proto/highlight.pro
@@ -43,6 +43,7 @@
 void set_context_in_highlight_cmd(expand_T *xp, char_u *arg);
 char_u *get_highlight_name(expand_T *xp, int idx);
 char_u *get_highlight_name_ext(expand_T *xp, int idx, int skip_cleared);
+int expand_highlight_group( char_u *pat, expand_T *xp, regmatch_T *rmp, char_u ***matches, int *numMatches);
 void free_highlight_fonts(void);
 void f_hlget(typval_T *argvars, typval_T *rettv);
 void f_hlset(typval_T *argvars, typval_T *rettv);