Add CI test to verify build output is in sync with source (.js vs .mjs)

With the introduction of a build step, where we take the source in .mjs
and build it into .js for non-ES Module systems, it's very important
that we remember to keep those two in sync.

That's what the CI job created in these changes ensure;

1. Run the build script
2. See if there are any pending git changes as a result

As long as there are no pending git changes, we're all fine and dandy,
or else we need to run the `npm run build` and commit the changes.

.github/workflows/verify.yml

@@ -82,3 +82,18 @@ jobs:
           deno-version: 'v0.23.0'
       - run: deno --version
       - run: deno test --allow-net=deno.land test/module-systems/deno-test.ts
+
+  build-output-sync:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: Setup Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
+      - name: Install, build and check git diff
+        run: |
+          npm ci
+          npm run build
+          git diff --quiet HEAD --