Add GitHub action for build workflow

This will publish the ml-header-generator package.

.github/workflows/build.yml

@@ -0,0 +1,35 @@
+name: build
+on:
+  release:
+    types: [created]
+  push:
+    branches:
+      - "**"
+    paths:
+      - "header-gen/**"
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./header-gen
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-node@v3
+        with:
+          node-version: "20.x"
+          registry-url: "https://npm.pkg.github.com"
+          cache: npm
+      - uses: microbit-foundation/npm-package-versioner-action@v1
+      - run: npm ci
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - run: npm run ci:update-version
+      - run: npm run build
+      - run: npm publish
+        if: github.ref == 'refs/heads/main' || github.event_name == 'release' && github.event.action == 'created'
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.GITHUB_TOKEN }}