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.

ElementX.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -108,15 +108,6 @@
         "version" : "7.30.2"
       }
     },
-    {
-      "identity" : "swift-argument-parser",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-argument-parser",
-      "state" : {
-        "revision" : "fddd1c00396eed152c45a46bea9f47b98e59301d",
-        "version" : "1.2.0"
-      }
-    },
     {
       "identity" : "swift-snapshot-testing",
       "kind" : "remoteSourceControl",
@@ -152,15 +143,6 @@
         "revision" : "12b5acf96d98f91d50de447369bd18df74600f1a",
         "version" : "1.9.6"
       }
-    },
-    {
-      "identity" : "yams",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/jpsim/Yams",
-      "state" : {
-        "revision" : "01835dc202670b5bb90d07f3eae41867e9ed29f6",
-        "version" : "5.0.1"
-      }
     }
   ],
   "version" : 2