Enable macOS GitHub Actions CI (#59)

* Enable macOS GitHub Actions CI

* Change macOS build command to build-only

* Add explicit macOS version list

Author: jmschonfeld

.github/workflows/pull_request.yml

@@ -13,6 +13,10 @@ jobs:
       linux_exclude_swift_versions: '[{"swift_version": "5.8"}, {"swift_version": "5.9"}, {"swift_version": "5.10"}]'
       windows_build_command: "swift build"
       windows_exclude_swift_versions: '[{"swift_version": "5.9"}, {"swift_version": "5.10"}]'
+      enable_macos_checks: true
+      macos_xcode_versions: '["16.3"]'
+      macos_build_command: "xcrun swift build"
+      macos_versions: '["sequoia"]'
   soundness:
     name: Soundness
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main