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/syntax.c b/src/syntax.c
index 34563aa..bbcf02b 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -6372,7 +6372,7 @@
 }
 
 /*
- * Handle command line completion for :match and :echohl command: Add "None"
+ * Handle command line completion for :match and :echohl command: Add "NONE"
  * as highlight group.
  */
     void