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.

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• ddb042aa - Add `sync` command

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Tests are failing but it doesn't look like it's due to my changes.

There's already:

ghcup upgrade
ghcup run --install --ghc recommended --cabal recommended --stack recommended --hls recommended

Do we need another command that does almost the same?

Maybe, maybe not.

In general I'm also not entirely sure whether "sync" is a good naming.

I'm leaning more towards extending ghcup upgrade and making its behavior smarter.

E.g. ghcup upgrade --tools=<ghc|cabal|stack|hls|installed|all> --channel=<recommended|latest>. But I'm not sold on the design yet.

Another question is... we actually would want to upgrade the ghcup binary first, before installing e.g. GHC. We need a way to preserve all cli arguments and then invoke the new binary again.

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