Setup GitHub actions

.github/workflows/build.yml

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+name: Build
+
+on:
+  workflow_call:
+
+jobs:
+  prepare:
+    name: Prepare
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Use Node.js
+        uses: actions/setup-node@v3
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'yarn'
+      - name: Install Yarn dependencies
+        run: yarn --immutable
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    needs:
+      - prepare
+    steps:
+      - uses: actions/checkout@v3
+      - name: Use Node.js
+        uses: actions/setup-node@v3
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'yarn'
+      - run: yarn --immutable --immutable-cache
+      - run: yarn build
+      - name: Require clean working directory
+        shell: bash
+        run: |
+          if ! git diff --exit-code; then
+            echo "Working tree dirty at end of job"
+            exit 1
+          fi

.github/workflows/main.yml

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+name: Main
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  check-workflows:
+    name: Check workflows
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Download actionlint
+        id: download-actionlint
+        run: bash <(curl https://raw.githubusercontent.com/rhysd/actionlint/7fdc9630cc360ea1a469eed64ac6d78caeda1234/scripts/download-actionlint.bash) 1.6.23
+        shell: bash
+      - name: Check workflow files
+        run: ${{ steps.download-actionlint.outputs.executable }} -color
+        shell: bash
+
+  build:
+    name: Build
+    uses: ./.github/workflows/build.yml
+
+  publish-to-gh-pages:
+    needs: build
+    if: github.ref == 'refs/heads/main'
+    name: Publish to the `gh-pages` branch
+    permissions:
+      contents: write
+    uses: ./.github/workflows/publish.yml
+    with:
+      destination_dir: ./

.github/workflows/publish.yml

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+name: Publish to GitHub Pages
+
+on:
+  workflow_call:
+    inputs:
+      destination_dir:
+        required: true
+        type: string
+
+jobs:
+  publish:
+    name: Publish to GitHub Pages
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Ensure `destination_dir` is not empty
+        if: ${{ inputs.destination_dir == '' }}
+        run: exit 1
+      - uses: actions/checkout@v3
+      - name: Setup Node
+        uses: actions/setup-node@v3
+        with:
+          node-version-file: '.nvmrc'
+      - run: yarn --immutable
+      - name: Run build script
+        run: yarn build
+      - name: Deploy to `${{ inputs.destination_dir }}` directory of `gh-pages` branch
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./public
+          destination_dir: ${{ inputs.destination_dir }}