Returns a new key path created by appending the given key path to this one.
- Xcode 9.0+
- Swift Standard Library
The key path to append.
A key path from the root of this key path to the value type of
Use this method to extend this key path to the value type of another key path. Calling
appending(path:) results in the same key path as if the given key path had been specified using dot notation.