Add the ability to parse a URL for `glab mr checkout`
Problem to solve
It's very easy to copy the current URL from the web GUI quickly, but it's a little harder to parse out the MR ID number unless you have some kind of script do it
Proposal
Allow URLs to be used as arguments for glab mr checkout. Ex: glab mr checkout https://gitlab.com/gitlab-org/cli/-/merge_requests/1234 would work the same as glab mr checkout 1234
Further details
I think some RegExs