Add multi-platform CI testing

Author: jwnimmer-tri

.github/workflows/continuous_integration.yml

@@ -38,7 +38,13 @@ jobs:
         run: |
           bazel run -- @buildifier --mode=check $(pwd)/*.b*z*l $(pwd)/example/*.b*z*l $(pwd)/workflows/*.b*z*l
   example_test:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        runner:
+          - macos-latest
+          - ubuntu-latest
+          - windows-latest
+    runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
       - uses: bazel-contrib/[email protected]