Add command line completions for installed and available versions.

When running `ghcup set ghc` and pressing tab, one should be able to
autocomplete the currently installed GHCs we have available.

Add an optparse applicative completer for install, rm, and set commands
which shows tags and versions. For installation, all are shown; while
for remove and set, only those installed are.

lib/GHCup/Utils.hs

@@ -808,3 +808,12 @@ getVersionInfo v' tool dls =
     % _head
     )
     dls
+
+
+-- Gathering monoidal values
+traverseFold :: (Foldable t, Applicative m, Monoid b) => (a -> m b) -> t a -> m b
+traverseFold f = foldl (\mb a -> (<>) <$> mb <*> f a) (pure mempty)
+
+-- | Gathering monoidal values
+forFold :: (Foldable t, Applicative m, Monoid b) => t a -> (a -> m b) -> m b
+forFold = flip traverseFold