Support swift-syntax from 600.0.0-latest (#165)

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

@@ -19,7 +19,7 @@ let package = Package(
   ],
   dependencies: [
     .package(url: "https://github.com/google/swift-benchmark", from: "0.1.0"),
-    .package(url: "https://github.com/apple/swift-syntax", "509.0.0"..<"511.0.0"),
+    .package(url: "https://github.com/apple/swift-syntax", "509.0.0"..<"601.0.0"),
     .package(url: "https://github.com/pointfreeco/swift-macro-testing", from: "0.2.0"),
     .package(url: "https://github.com/pointfreeco/xctest-dynamic-overlay", from: "1.0.0"),
   ],