Support swift-syntax from 600.0.0-latest (pointfreeco#858)

The Xcode 16 beta generates macro projects using these swift-syntax
snapshots. Luckily things seem to be backwards compatible, so we can
expand our supported range.

Author: stephencelis

Package.resolved

@@ -21,7 +21,7 @@ let package = Package(
     ),
   ],
   dependencies: [
-    .package(url: "https://github.com/apple/swift-syntax", "508.0.1"..<"511.0.0")
+    .package(url: "https://github.com/apple/swift-syntax", "508.0.1"..<"601.0.0")
   ],
   targets: [
     .target(