Reduce String CoW

By introducing an intermediate proxy class for the array subscript
operator for String and CharString we can control better when CowData
will actually CoW.

This should improve performance of String usage for most cases.

modules/gdnative/gdnative_api.json

@@ -4755,7 +4755,7 @@
       },
       {
         "name": "godot_string_operator_index",
-        "return_type": "wchar_t *",
+        "return_type": "const wchar_t *",
         "arguments": [
           ["godot_string *", "p_self"],
           ["const godot_int", "p_idx"]