Improve dictionary printing to avoid confusion with arrays

- Add leading and trailing spaces within dictionaries, as the `{}`
  characters are hard to distinguish from `[]` on some fonts.
  This is especially helpful with empty arrays and dictionaries.

modules/gdscript/tests/scripts/runtime/features/stringify.out

@@ -21,7 +21,7 @@ hello/world
 RID(0)
 Node::get_name
 Node::[signal]property_list_changed
-{"hello":123}
+{ "hello": 123 }
 ["hello", 123]
 [255, 0, 1]
 [-1, 0, 1]