Pre-run checks

[systay]

We should do pre-flight checks instead of failing part way through. This could mean checking that the necessary directories are in place, that the needed gh extensions have been loaded and so on.

That way, a nice little summary can be prepared with things that need to be fixed before the tool can run.

To check that the needed extension is loaded:

>  gh extension list
NAME          REPO                       VERSION
gh dash       dlvhdr/gh-dash             v3.11.0
gh milestone  valeriobelli/gh-milestone  v2.1.0