vim-patch:8.1.1693: syntax coloring and highlighting is in one big file (#17721)

Problem:    Syntax coloring and highlighting is in one big file.
Solution:   Move the highlighting to a separate file. (Yegappan Lakshmanan,
            closes vim/vim#4674)

f9cc9f209e

Name the new file highlight_group.c instead.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/buffer.c

@@ -51,6 +51,7 @@
 #include "nvim/getchar.h"
 #include "nvim/hashtab.h"
 #include "nvim/highlight.h"
+#include "nvim/highlight_group.h"
 #include "nvim/indent.h"
 #include "nvim/indent_c.h"
 #include "nvim/main.h"