refactor: iwyu #31637

Result of `make iwyu` (after some "fixups").

src/nvim/highlight_group.c

@@ -1,5 +1,6 @@
 // highlight_group.c: code for managing highlight groups
 
+#include <assert.h>
 #include <ctype.h>
 #include <stdbool.h>
 #include <stdint.h>
@@ -31,6 +32,7 @@
 #include "nvim/garray_defs.h"
 #include "nvim/gettext_defs.h"
 #include "nvim/globals.h"
+#include "nvim/grid_defs.h"
 #include "nvim/highlight.h"
 #include "nvim/highlight_group.h"
 #include "nvim/lua/executor.h"