Widen swift-syntax dependency to 508.0.1..<510.0.0 (#795)

Making `assertInlineSnapshot` simply unavailable when another dependency
locks to 508.0.1. I think there are too many significant changes in
swift-syntax 509 to support the function in earlier versions.

Author: stephencelis

Package.swift

@@ -21,7 +21,7 @@ let package = Package(
     ),
   ],
   dependencies: [
-    .package(url: "https://github.com/apple/swift-syntax.git", from: "509.0.0")
+    .package(url: "https://github.com/apple/swift-syntax.git", "508.0.1"..<"510.0.0")
   ],
   targets: [
     .target(