Show warning when sidekick init would downgrade the cli

[passsy]

In the past, we used sidekick init to "upgrade" a sidekick cli to the latest template version.

Now it may happen, that a user calls sidekick init with an outdated sidekick version, basically downgrading the sidekick package.

We can prevent this by checking the version in pubspec.yaml and compare it to the version of sidekick_core.

[danielmolnar]

Isn't this being prevented by the following bit within UpdateCommand?

    if (versionToInstall <= currentSidekickCliVersion) {
      print('No need to update because you are already using the '
          'latest sidekick cli version.');
      return;
    }

[passsy]

The UpdateCommand is not used, it's the InitCommand