highlight: refactor to use stateful representation

This allows us to keep track of the source higlight groups,
and not only the final combined highlights.

src/nvim/main.c

@@ -183,6 +183,7 @@ void early_init(void)
   eval_init();          // init global variables
   init_path(argv0 ? argv0 : "nvim");
   init_normal_cmds();   // Init the table of Normal mode commands.
+  highlight_init();
 
 #if defined(HAVE_LOCALE_H)
   // Setup to use the current locale (for ctype() and many other things).