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/Tests/DesignKitTests.swift

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