This PR updates the `pr-title` CI check to enforce that the commit
message does not start with a capital letter followed by a non-capital
letter.
This should ensure that messages do not start with a capitalized word,
but allow messages that start with an acronym.
Also turn this into a proper check, run when a PR is opened or edited.
I took the liberty to rename the workflow file and name, so that one
doesn't have to look inside to guess what the workflow is doing.
