Support swift-syntax from 600.0.0-latest (#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.

Package.resolved

@@ -3,10 +3,10 @@
     {
       "identity" : "swift-syntax",
       "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-syntax.git",
+      "location" : "https://github.com/apple/swift-syntax",
       "state" : {
-        "revision" : "08a2f0a9a30e0f705f79c9cfaca1f68b71bdc775",
-        "version" : "510.0.0"
+        "revision" : "303e5c5c36d6a558407d364878df131c3546fad8",
+        "version" : "510.0.2"
       }
     }
   ],

Package.swift

@@ -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(