[lua] respect -D lua-vanilla in StringTools (see #7589)

std/StringTools.hx

@@ -471,7 +471,11 @@ class StringTools {
 		#elseif hl
 		return @:privateAccess s.bytes.getUI16(index << 1);
 		#elseif lua
-		return lua.lib.luautf8.Utf8.byte(s,index+1);
+			#if lua_vanilla
+			return lua.NativeStringTools.byte(s,index+1);
+			#else
+			return lua.lib.luautf8.Utf8.byte(s,index+1);
+			#end
 		#else
 		return untyped s.cca(index);
 		#end