Minor Swiftly cleanup

[matthewbastien]

Description

A few minor changes to the Swiftly integration:

• Move Swiftly toolchains to the top of the toolchain selection quick pick. If Swiftly is installed and functioning then users most likely want to use it over the other options.
• Hide the Swiftly install commands from the command palette if Swiftly is not available or doesn't support it.
• Rename listAvailableToolchains() to list() so that it matches the Swiftly command and doesn't get confused with listAvailable().
• Clean up some of the JSDoc comments above Swiftly functions.

Tasks

• [ ] Required tests have been written
• [ ] Documentation has been updated
• [ ] Added an entry to CHANGELOG.md if applicable

[matthewbastien]

Swift 6.0 Windows jobs keep failing on unrelated issues. Merging this despite the CI failures.