Fix CI for macOS 14.4 build with latest SDK (#7)

The default macOS-14 platform didn't use the latest SDK and so some minor `UndoManager` assertions produced build errors.

Author: DivineDominion

.github/workflows/ci.yml

@@ -18,5 +18,9 @@ jobs:
     runs-on: macOS-14
     steps:
       - uses: actions/checkout@v4
+      - name: Xcode Select Version
+        uses: mobiledevops/xcode-select-version-action@v1
+        with:
+          xcode-select-version: 15.4
       - name: Test
         run: set -o pipefail && swift test | xcbeautify --renderer github-actions