highlight: extract low-level highlight logic from syntax, ui

src/nvim/api/vim.c

@@ -21,6 +21,7 @@
 #include "nvim/vim.h"
 #include "nvim/buffer.h"
 #include "nvim/file_search.h"
+#include "nvim/highlight.h"
 #include "nvim/window.h"
 #include "nvim/types.h"
 #include "nvim/ex_docmd.h"