Add CI test verifying Mustache works with CommonJS

This is a precursor to introducing a build step that will change what
we expose from this package. Better off writing some tests to verify
existing projects with different module systems continue to work as
expected.

.github/workflows/verify.yml

@@ -38,3 +38,22 @@ jobs:
         run: |
           npm install mocha@3 chai@3
           npm run test-unit
+
+  common-js-usage:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: Setup Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
+      - name: Package, install and test
+        run: |
+          export ARCHIVE_FILENAME=$(npm pack | tail -n 1)
+          export UNPACK_DESTINATION=$(mktemp -d)
+          mv $ARCHIVE_FILENAME $UNPACK_DESTINATION
+          cp test/module-systems/commonjs-test.js $UNPACK_DESTINATION
+          cd $UNPACK_DESTINATION
+          npm install $ARCHIVE_FILENAME
+          node commonjs-test.js

test/module-systems/.eslintrc

@@ -0,0 +1,6 @@
+{
+  "extends": "../.eslintrc",
+  "parserOptions": {
+    "ecmaVersion": 2017
+  }
+}

test/module-systems/commonjs-test.js

@@ -0,0 +1,12 @@
+const assert = require('assert');
+const mustache = require('mustache');
+
+const view = {
+  title: 'Joe',
+  calc: () => 2 + 4
+};
+
+assert.strictEqual(
+  mustache.render('{{title}} spends {{calc}}', view),
+  'Joe spends 6'
+);