Relax swift-syntax requirement (#263)

Relaxes the swift-syntax version between 509 and 601, this is required for projects using a new version of swift-syntax.

Author: luispadron

Package.resolved

@@ -1,5 +1,5 @@
 {
-  "originHash" : "479104e2c09bb55577e8974f6216f123d295af4fd9984b1f19eef2a4c5eb3067",
+  "originHash" : "8143c64c0e8fef46075546224dfd031a884a7ab571cd335d893ee4386799ecca",
   "pins" : [
     {
       "identity" : "swift-argument-parser",
@@ -15,8 +15,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/apple/swift-syntax.git",
       "state" : {
-        "revision" : "2bc86522d115234d1f588efe2bcb4ce4be8f8b82",
-        "version" : "510.0.3"
+        "revision" : "0687f71944021d616d34d922343dcef086855920",
+        "version" : "600.0.1"
       }
     }
   ],

Package.swift

@@ -18,7 +18,7 @@ let package = Package(
         .executable(name: "knit-cli", targets: ["knit-cli"]),
     ],
     dependencies: [
-        .package(url: "https://github.com/apple/swift-syntax.git", from: "510.0.2"),
+        .package(url: "https://github.com/apple/swift-syntax.git", "510.0.2"..<"601.0.0"),
         .package(url: "https://github.com/apple/swift-argument-parser", from: "1.4.0"),
     ],
     targets: [