Help modders deal with object invalidation (#14769)

* Skip invalid objects in raycasts
* Add `ObjectRef:is_valid` method
* Add object inside radius / area iterators which skip invalid objects
* Update docs to clarify object invalidation and how to deal with it

---------

Co-authored-by: sfan5 <sfan5@live.de>

src/script/lua_api/l_object.h

@@ -67,6 +67,9 @@ private:
 	// remove(self)
 	static int l_remove(lua_State *L);
 
+	// is_valid(self)
+	static int l_is_valid(lua_State *L);
+
 	// get_pos(self)
 	static int l_get_pos(lua_State *L);