Add support for processing evolution proposals for additional projects

[stmontgomery]

We recently launched a Testing Workgroup in Swift, and as part of that, we're beginning to place proposals focused on Swift Testing in a new /proposals/testing subdirectory of the swift-evolution repo.

As of swiftlang/swift-evolution#2721, that subdirectory now exists and contains historical proposals which were approved before Swift Testing began formally participating in the evolution process.

Shortly after that landed, someone realized swift-evolution-metadata-extractor had broken and was no longer successfully processing proposal updates. @hborla landed a quick fix for that issue in #48, which simply ignored all content in the /proposals/testing directory.

This issue tracks adding formal support for including the proposals in that directory and showing them on the Swift Evolution Dashboard. That includes:

• Finding proposals in the /proposals/testing subdirectory, or perhaps in any subdirectory of /proposals (to future-proof, if desired).
• Adjusting anything else which may be necessary to successfully parse the header of these proposals. Hopefully there isn't anything materially different, but if there is, that should be accommodated. (Swift Testing uses a distinct template from Swift and SwiftPM, although its header is quite similar.)
• Ensuring that Swift Testing's custom "ST-NNNN" prefix is handled correctly. (It's intentionally different than "SE-NNNN".)

[stmontgomery]

I'm interested in contributing this, but @dempseyatgithub if you'd rather handle this yourself feel free. I do think we'd prefer to see this happen relatively soon if possible (assuming there are no objections to the idea).

[dempseyatgithub]

I think there are a few things to evaluate here, including whether having all evolution proposals across all projects (currently just adding Swift Testing, but potentially Foundation, other projects over time) in a single json file is the best approach. A lot of this would be driven by the evolution dashboard and how it is going to display proposals of different projects.

[dempseyatgithub]

I've started work on extending the extractor to handle multiple projects.