Add version-specific manifests for Swift 5.4 and 5.5

[compnerd]

This changes the build to track main, which is required when building against the latest toolchain. This allows us to build SwiftSyntaxHighlighter on Windows with the latest toolchain.

[compnerd]

@mattt I'm not sure whats the best way to handle this. Tracking releases via branch makes some sense I think. That is you could have a release/5.3, release/5.4, release/5.5 to track the released branches and have main track main. This was the last piece that allows me to build SwiftSyntaxHighlighter on Windows.

I think that with this change merged, it might even be possible to setup CI to keep the package building on Windows.

[mattt]

@compnerd What if we did the following?

• Keep Package.swift the same, targeting the latest supported version of Swift (5.3, for now)
• Create Swift version-specific manifests (Package@swift-5.4.swift and Package@swift-5.5.swift)
  • In each version-specific manifest, target a release branch (so release/5.4 and release/5.5, respectively)
• Remove and ignore Package.resolved to allow CI to pass
• Update CI to run matrix of supported Swift versions
• Going forward, whenever a new major / minor version of Swift is released, we do the following:
  • Demote the existing Package.swift to a version-specific manifest (so Package@swift-5.4.swift)
  • Promote that version-specific manifest to Package.swift
  • Convert the branch reference in Package.swift to a stable tag

Would something like that meet your needs?

[compnerd]

I think that should work. Really, ultimately, what Im looking for is being able to build SwiftSyntaxHighlighter with 5.4+ so that it is available on Windows as well to get swift-doc.

[mattt]

@compnerd This is now available in version 1.2.2