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/object.h

@@ -658,6 +658,7 @@ public:
 	void call_multilevel(const StringName &p_name, VARIANT_ARG_LIST); // C++ helper
 
 	void notification(int p_notification, bool p_reversed = false);
+	String to_string();
 
 	//used mainly by script, get and set all INCLUDING string
 	virtual Variant getvar(const Variant &p_key, bool *r_valid = NULL) const;