lua: enabling string lib

firmware/controllers/lua/lua.cpp

@@ -141,6 +141,7 @@ static int lua_setTickRate(lua_State* l) {
 static void loadLibraries(LuaHandle& ls) {
 	constexpr luaL_Reg libs[] = {
 		{ LUA_GNAME, luaopen_base },
+		{ LUA_STRLIBNAME, luaopen_string },
 		{ LUA_MATHLIBNAME, luaopen_math },
 	};