[windows] Provide more explicit means of uninstallation

Uninstallation is generally scary. GHCup already provides means to uninstall installed tools (such as GHC, cabal, ...), but Windows uninstallation would be a little more than that. So after implementing #135 (closed) which is rather well-defined, we need to deal with the following in addition

1. uninstall MSYS2 if the user wants (remember, we allow to hook in an existing install)
2. clean up several environment variables that were set (this may affect e.g. cabal from another install, since we set CABAL_DIR globally to avoid issues with MAX_PATH)
3. clean up desktop files
4. maybe clean up .cabal

There could be multiple ways to achieve this:

1. integrate uninstallation as an option in the default bootstrap script that's also used for installation
2. create a separate script and place it on the desktop (or somewhere else?), which will carry out the uninstallation interactively