More specific function keyword high-lighting

I learnt about this group by reading the nvim-treesitter source [1].

[1]: see 'lua/nvim-treesitter/highlight.lua'

test/highlight/keywords.tig

@@ -5,9 +5,9 @@ let
   /* <- keyword */
 
   function f() : int = a
-  /* <- keyword */
+  /* <- keyword.function */
   primitive g()
-  /* <- keyword */
+  /* <- keyword.function */
 
   import "lib.tih"
   /* <- keyword */