GDScript: Allow string keys on Lua-style dictionaries

Which is useful when the key isn't a valid identifier, such as keys with
spaces or numeric keys.

modules/gdscript/tests/scripts/parser/features/dictionary_lua_style.gd

@@ -0,0 +1,9 @@
+func test():
+	var lua_dict = {
+		a = 1,
+		"b" = 2, # Using strings are allowed too.
+		"with spaces" = 3, # Especially useful when key has spaces...
+		"2" = 4, # ... or invalid identifiers.
+	}
+
+	print(lua_dict)