Add action to check that dist is up to date (#8)

.github/workflows/check-dist.yml

@@ -0,0 +1,44 @@
+# `dist/index.js` is a special file in Actions.
+# When you reference an action with `uses:` in a workflow,
+# `index.js` is the code that will run.
+# For our project, we generate this file through a build process
+# from other source files.
+# We need to make sure the checked-in `index.js` actually matches what we expect it to be.
+name: Check dist/
+
+on: [push, pull_request]
+
+jobs:
+  check-dist:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20.x
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Rebuild the dist/ directory
+        run: npm run prepare
+
+      - name: Compare the expected and actual dist/ directories
+        run: |
+          if [ "$(git diff --ignore-space-at-eol dist/ | wc -l)" -gt "0" ]; then
+            echo "Detected uncommitted changes after build.  See status below:"
+            git diff
+            exit 1
+          fi
+        id: diff
+
+      # If index.js was different than expected, upload the expected version as an artifact
+      - uses: actions/upload-artifact@v4
+        if: ${{ failure() && steps.diff.conclusion == 'failure' }}
+        with:
+          name: dist
+          path: dist/