perf(treesitter): avoid string lookup of highlight name in hot loop

These numbers are guaranteed to be stable even if you do "highlight
clear" (all attributes disappear, but not the id to name mapping itself)

runtime/lua/vim/treesitter/highlighter.lua

@@ -85,8 +85,10 @@ function TSHighlighterQuery.new(lang, query_string)
         hl = _link_default_highlight_once(lang .. hl, hl)
       end
 
-      rawset(table, capture, hl)
-      return hl
+      local id = a.nvim_get_hl_id_by_name(hl)
+
+      rawset(table, capture, id)
+      return id
     end
   })