refactor(api): give "hl_group" more accurate _meta type

These can either be number or string in lua, so we can specify this
directly as "number|string".

scripts/gen_eval_files.lua

@@ -88,6 +88,7 @@ local API_TYPES = {
   LuaRef = 'function',
   Dictionary = 'table<string,any>',
   Float = 'number',
+  HLGroupID = 'number|string',
   void = '',
 }