Investigate string function

This function needed a cast in order for master to compile, there was a PR for introducing this. @karroffel mentioned he wanted a closer look so just raising this to make sure it's not forgotten.

`core/String.cpp`
```
wchar_t &String::operator[](const int idx) {
return *const_cast(godot::api->godot_string_operator_index(&_godot_string, idx));
}
```