Allow overriding how scripted objects are converted to strings

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()`

core/variant.cpp

@@ -1582,7 +1582,7 @@ Variant::operator String() const {
 					};
 				};
 #endif
-				return "[" + _get_obj().obj->get_class() + ":" + itos(_get_obj().obj->get_instance_id()) + "]";
+				return _get_obj().obj->to_string();
 			} else
 				return "[Object:null]";