Break out DesignKit into its own sub-package (#459)

We'll be using https://github.com/vector-im/compound-ios eventually so this tidies up the Xcode project dependencies in the mean time.

Package.resolved

@@ -1,14 +1,5 @@
 {
   "pins" : [
-    {
-      "identity" : "element-design-tokens",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/vector-im/element-design-tokens.git",
-      "state" : {
-        "revision" : "63e40f10b336c136d6d05f7967e4565e37d3d760",
-        "version" : "0.0.3"
-      }
-    },
     {
       "identity" : "swift-argument-parser",
       "kind" : "remoteSourceControl",
@@ -18,15 +9,6 @@
         "version" : "1.2.0"
       }
     },
-    {
-      "identity" : "swiftui-introspect",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/siteline/SwiftUI-Introspect.git",
-      "state" : {
-        "revision" : "f2616860a41f9d9932da412a8978fec79c06fe24",
-        "version" : "0.1.4"
-      }
-    },
     {
       "identity" : "yams",
       "kind" : "remoteSourceControl",