api: add API for themes

co-author: hlpr98 <hlpr98@gmail.com> (dict2hlattrs function)

orange is sus??

NOVEMBER DAWN

erase the lie that is redraw_later()

src/nvim/types.h

@@ -21,7 +21,7 @@ typedef int handle_T;
 // absent callback etc.
 typedef int LuaRef;
 
-typedef uint64_t NS;
+typedef handle_T NS;
 
 typedef struct expand expand_T;