Add workflow job to label PRs with conflicts

[ferdnyc]

A purely take-it-or-leave-it PR, to add a tool that I find helpful in upstream repos. The workflow added by this PR will scan each open PR in the repo, after every change to the source or target branches, and check whether merge conflicts have appeared (or disappeared).

If conflicts are found, it will then both:

1. Add a comment to the PR, requesting that the conflicts be addressed.
2. Label the PR with the selected label (Currently set to "conflicts"), so that the presence of conflicts is visible in the PR overview listing.

If prior conflicts have been resolved, it will:

1. Comment on the PR, thanking the submitter for addressing the issue.
2. Remove the "conflicts" label.

It's a handy replacement for a feature GitHub should provide themselves, but inexplicably still do not.

IMPORTANT PREREQUISITE

This workflow will not work, unless the label it is configured to use is created manually before merging it.

As I said, the currently-configured label name is conflicts (all-lowercase), which is how the label should be created at https://github.com/pydot/pydot/labels (unless we decide to change it). Color is irrelevant, though I suggest #ff0000 red so that it stands out clearly in the PR list.