👷 add Clang/Windows step

.github/workflows/windows_clang.yml

+name: Windows
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: windows-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: install Clang
+        run: choco install -y llvm
+      - name: cmake
+        run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On
+      - name: build
+        run: cmake --build build --parallel 10
+      - name: test
+        run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure