Add GitHub workflow for JavaScript building

extensions/markdown/.github/workflows/build.yml

@@ -0,0 +1,17 @@
+name: Build JavaScript assets
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@master
+      - uses: flarum/action-build@master
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+