Add a `sourcekit-lsp version` command that outputs the SourceKit-LSP version

[ahoppen]

Ideally, this would output the corresponding Swift version (eg. 6.0) and the commit hash from which it was built. Maybe we can use a build plugin to get the commit hash and the Swift version from the current branch name.

[ahoppen]

Synced to Apple’s issue tracker as rdar://129223520

[ahoppen]

We should be able to do this by

• Reading Context.gitInformation?.currentCommit in the package manifest
• Passing that information into a C modules using -D
• Calling a function in the C modules to return the info

Context.gitInformation?.currentCommit is only available with swift-tools-version 6.0, so we can only do this after we require Swift 6 (because I don’t want to have two different package manifests to maintain just for this feature).