Add GitHub Actions CI for pull requests

[jfrost-mo]

Fixes #33

Also has a second actions job to check that the markdown rendering of the standard names is up to date. It would be fairly easy to extend this to add a commit to the PR updating it if it isn't, but I haven't added that here as it wasn't really the focus.

[nusbaume]

Thanks so much for opening this PR! I didn't even realize the Met Office was interested in this project, but we'll happily take any help we can get.

Anyways, I tried testing the Actions workflow and found that one step related to xmllint was missing for each job. However, once those changes are made and the merge conflict is resolved I think this PR will be good to go, at least on my end (i.e. the workflow tests all behaved as expected in my testing environment). Of course If you need any assistance with the requested changes just let me know.

[jfrost-mo]

I've added xmllint installation to the workflow, and rebased onto main to fix the merge conflicts.
As context, my interest in this project stems from its use in JEDI.

[nusbaume]

Looks good to me now, thanks again!

[climbfuji]

Thanks very much for adding this capability, @jfrost-mo !