add github action build script

.github/workflows/build.yml

@@ -0,0 +1,54 @@
+name: build
+
+permissions:
+  contents: write
+  repository-projects: write
+
+on:
+  workflow_dispatch:
+    branches: [dev]
+
+jobs:
+  build:
+    name: bls-eth-wasm
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: node
+      uses: actions/setup-node@v4
+      with:
+        node-version: "20"
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    - run: |
+        git pull
+        cur=$(git rev-parse --abbrev-ref HEAD)
+        echo "current branch is $cur."
+        if [ "$cur" != "dev" ]; then
+          echo "err : not dev branch"
+          exit 1
+        fi
+        git config user.name github-actions
+        git config user.email github-actions@github.com
+        git clone https://github.com/emscripten-core/emsdk.git
+        cd emsdk/
+        ./emsdk install latest
+        ./emsdk activate latest
+        source ./emsdk_env.sh
+        emcc -v
+        cd ..
+        git submodule update --init --recursive
+        make -C src
+        cd browser
+        npm install -D webpack webpack-cli
+        npx webpack
+        cd ..
+        npm test
+        sha256sum src/bls_c.js browser/bls.js
+        git add -f src/bls_c.js browser/bls.js
+        changes=$(git diff --staged --numstat | wc -l)
+        if [[ $changes -gt 0 ]]
+        then
+          git commit -m "build: bls_c.js is generated by GitHub Action"
+          git push origin dev
+        fi