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()`

modules/gdscript/gdscript.h

@@ -251,6 +251,7 @@ public:
 	Variant debug_get_member_by_index(int p_idx) const { return members[p_idx]; }
 
 	virtual void notification(int p_notification);
+	String to_string(bool *r_valid);
 
 	virtual Ref<Script> get_script() const;