Make Command extend Shellbox's UnboxedCommand. Only a few MediaWiki-
specific features remain in the subclass.
Also add BoxedCommand abstraction and Shellbox client.
The Result alias didn't actually work, I just had to change the return
type hint.
Bug: T260330
Change-Id: Iff7428e4c5fe3959a5cda8e113f223caa0976fc1
