ci: initial build workflow set up

.github/workflows/build.yaml

@@ -0,0 +1,44 @@
+name: Build
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: macos-latest
+
+    strategy:
+      matrix:
+        platform: [ios, macos, watchos, tvos]
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+
+    - name: Set up Swift
+      uses: fwal/setup-swift@v1
+      with:
+        swift-version: '5.9'
+
+    - name: Build for ${{ matrix.platform }}
+      run: |
+        case ${{ matrix.platform }} in
+          ios)
+            swift build --destination 'platform=iOS Simulator,OS=14.0,name=iPhone 11'
+            ;;
+          macos)
+            swift build --destination 'platform=macOS,arch=x86_64'
+            ;;
+          watchos)
+            swift build --destination 'platform=watchOS Simulator,OS=7.0,name=Apple Watch Series 5'
+            ;;
+          tvos)
+            swift build --destination 'platform=tvOS Simulator,OS=14.0,name=Apple TV'
+            ;;
+        esac