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/core_string_names.h

@@ -62,6 +62,7 @@ public:
 	StringName _iter_next;
 	StringName _iter_get;
 	StringName get_rid;
+	StringName _to_string;
 #ifdef TOOLS_ENABLED
 	StringName _sections_unfolded;
 #endif