CI: add Windows coverage

Add Windows to the CI matrix for additional coverage.

.github/workflows/ci.yml

@@ -31,3 +31,30 @@ jobs:
         uses: actions/checkout@v1
       - name: Build and Test
         run: swift test --enable-test-discovery
+
+  windows:
+    runs-on: windows-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: seanmiddleditch/gha-setup-vsdevenv@master
+
+      - name: Install swift-5.4
+        run: |
+          Install-Binary -Url "https://swift.org/builds/swift-5.4-release/windows10/swift-5.4-RELEASE/swift-5.4-RELEASE-windows10.exe" -Name "installer.exe" -ArgumentList ("-q")
+      - name: Set Environment Variables
+        run: |
+          echo "SDKROOT=C:\Library\Developer\Platforms\Windows.platform\Developer\SDKs\Windows.sdk" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+          echo "DEVELOPER_DIR=C:\Library\Developer" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+      - name: Adjust Paths
+        run: |
+          echo "C:\Library\Swift-development\bin;C:\Library\icu-67\usr\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+          echo "C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+      - name: Install Supporting Files
+        run: |
+          Copy-Item "$env:SDKROOT\usr\share\ucrt.modulemap" -destination "$env:UniversalCRTSdkDir\Include\$env:UCRTVersion\ucrt\module.modulemap"
+          Copy-Item "$env:SDKROOT\usr\share\visualc.modulemap" -destination "$env:VCToolsInstallDir\include\module.modulemap"
+          Copy-Item "$env:SDKROOT\usr\share\visualc.apinotes" -destination "$env:VCToolsInstallDir\include\visualc.apinotes"
+          Copy-Item "$env:SDKROOT\usr\share\winsdk.modulemap" -destination "$env:UniversalCRTSdkDir\Include\$env:UCRTVersion\um\module.modulemap"
+      - name: Build and Test
+        run: swift test --enable-test-discovery