Add an initial GitHub Actions config to run Safari stable

[gsnedders]

We'll likely want to refactor this into a reusable workflow, especially for Safari Technology Preview, but potentially for any other browsers we want to run on GitHub Actions.

That said, let's start with the simple singular case and make sure this works before adding more complexity.

[gsnedders]

Did it work? Is there an artifacts upload somewhere to inspect?

https://github.com/gsnedders/web-platform-tests/actions/runs/9980202391

Assuming this works it LGTM. For preference I'd have all the steps that are just running shell commands as a single shell script that can be run outside of CI rather than inlining them into the workflow file, but it's not a blocker.

This largely just matches what Azure Pipelines does, albeit with everything in the workflow instead of split into many actions. This is also arguably overly simplistic, because we really want the workflow to be reusable for multiple browsers, which means we need steps to be conditional — but it seemed easiest to inline everything and iterate from here.

(It was, in actuality, ultimately derived from an automatically converted Azure Pipelines config.)

[jgraham]

OK, well happy for this to be merged as-is.