Add management of Swift development toolchain snapshots

I'd love for Xcodes to be configurable to automatically download and install Swift development toolchain snapshots from https://www.swift.org/download/.

For example, I have a project running in Swift 5.9 that could be enhanced using features from Swift 5.10, which is still in development. I would like to keep Xcodes running in the background and have it automatically download and install the latest Swift 5.10 toolchain so that I always have the latest snapshot available for testing 5.10 builds of my project.

Implementing this feature would probably involve parsing the Swift downloads page to locate the section headings such as "Trunk Development (main)", "Swift 5.10 Development", and "Swift 5.9 Development". The Xcodes UI would then let me select one or more of these sections, and see the toolchains from that section that are already installed or available to be installed. There would also be an option to install the latest snapshot and delete older snapshots automatically. Perhaps there would also be an option to "pin" a snapshot so that it would not be deleted.

A feature like this would make working with prerelease versions of Swift as easy as working with prerelease versions of Xcode.

Start by reviewing the Swift downloads page sections named in the issue, including Trunk Development (main) and Swift 5.10 Development. Then trace the Xcodes UI and existing toolchain installation flow to determine how section selection, latest-snapshot installation, cleanup, and pinning would fit; the issue names no files or tests.