Add GitHub Actions job to execute tests

.github/workflows/showdown-highlight-tests.yml

@@ -0,0 +1,21 @@
+name: showdown-highlight-tests
+
+on: [push, pull_request]
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [18.x, 20.x, 22.x]
+        # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+      - run: npm install
+      - run: npm test