Add GitHub Actions, fix Windows 3.1 & mswin

[MSP-Greg]

1. Actions CI - similar to the code in PR #178, but only the workflow file. Updated Rubies, etc. Note that the coverage specific steps all failed with the other changes, so I added if: false to skip them for now.

2. Added one skip. Frustrating. Sometimes Windows get squirrelly when a file is opened more than once, which the skipped spec does. But, running locally on Windows 11 and Ruby master, I could not repo the failure. Tried several things, and none fixed the CI failure. Maybe.later.

3. Publicly available Windows Rubies 2.4 thru 3.0 are all built with the MSYS2 mingw tool chain. Starting with Windows Ruby 3.1, builds are using the MSYS2 ucrt tool chain. The Windows fix only affects 3.1 and mswin builds.

Feel free to grab the Windows fix and add to PR #178.

[MSP-Greg]

Re the second point above, the error shown is

     ChildProcess::LaunchError:
       Access is denied. (5)

I assumed that was a file error. I rewrote the test to use a UDP socket to pass the pid needed for the spec, and it still had the same error in Actions. But, as with the existing spec, the UDP version also passed locally on Windows 11 desktop. So, I'm not sure why it's failing on the Actions CI runners.

[sds]

Thank you! Superseded by #175.