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/vim.h b/src/vim.h
index e0c40c9..2ebc8fe 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -846,6 +846,7 @@
 #define EXPAND_DIRS_IN_CDPATH	59
 #define EXPAND_SHELLCMDLINE	60
 #define EXPAND_FINDFUNC		61
+#define EXPAND_HIGHLIGHT_GROUP  62
 
 
 // Values for exmode_active (0 is no exmode)