In order to avoid major disruptions, I introduced default value to
fallback to but in longer term we probably should deprecated it.
Bug: T285105
Change-Id: I81d9ece769c4942ef2ca390a40ff9d2c24c9ece4
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
