Add `sync` command

This is my first take at a sync command which upgrades GHCup (like upgrade does) and installs all tools at the recommended version.

Closes #169 (nice)

Notes:

• The error-handling is pretty rudimentary. I'll have to thread all the ExitCodes together somehow. Advice on this would be appreciated; I'm not super familiar with the libraries here.
• Right now, it only updates tools; it won't install new ones. It would be nice if it supported this.
• Should this command have all the options of the upgrade and install commands? I think a nice set of defaults is fine, at least for a first release of this feature.