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.

DesignKit/Package.resolved

@@ -0,0 +1,23 @@
+{
+  "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" : "swiftui-introspect",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/siteline/SwiftUI-Introspect.git",
+      "state" : {
+        "revision" : "f2616860a41f9d9932da412a8978fec79c06fe24",
+        "version" : "0.1.4"
+      }
+    }
+  ],
+  "version" : 2
+}

DesignKit/Package.swift

@@ -0,0 +1,29 @@
+// swift-tools-version: 5.7
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "DesignKit",
+    platforms: [
+        .iOS(.v14)
+    ],
+    products: [
+        .library(name: "DesignKit", targets: ["DesignKit"])
+    ],
+    dependencies: [
+        .package(url: "https://github.com/vector-im/element-design-tokens.git", exact: "0.0.3"),
+        .package(url: "https://github.com/siteline/SwiftUI-Introspect.git", from: "0.1.4")
+    ],
+    targets: [
+        .target(name: "DesignKit",
+                dependencies: [
+                    .product(name: "DesignTokens", package: "element-design-tokens"),
+                    .product(name: "Introspect", package: "SwiftUI-Introspect")
+                ],
+                path: "Sources"),
+        .testTarget(name: "DesignKitTests",
+                    dependencies: ["DesignKit"],
+                    path: "Tests")
+    ]
+)

DesignKit/Tests/DesignKitTests.swift

@@ -0,0 +1,8 @@
+@testable import DesignKit
+import XCTest
+
+final class DesignKitTests: XCTestCase {
+    func testExample() throws {
+        XCTAssert(true)
+    }
+}