Add a workflow to verify that dist/ is up to date

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/verify-dist.yml

@@ -0,0 +1,26 @@
+name: Verify that dist is compiled
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: npm ci
+      - run: npm run lint
+      - run: npm run prepare
+      - name: Verify that `dist/index.js` is up to date
+        shell: bash
+        run: |
+          # `ncc` might produce output with mixed line endings
+          test -z "$(git diff -w HEAD | tee diff.txt)" || {
+            echo 'Files changed after `npm run package`'
+            cat diff.txt
+            exit 1
+          }