solves #26796

- ADD `String to_string()` method to Object which can be overriden by `String _to_string()` in scripts
- ADD `String to_string(r_valid)` method to ScriptInstance to allow langauges to control how scripted objects are converted to strings
- IMPLEMENT to_string for GDScriptInstance, VisualScriptInstance, and NativeScriptInstance
- ADD Documentation about `Object.to_string` and `Object._to_string`
- Changed `Variant::operator String` to use `obj->to_string()`