feat: add prettier workflow

.github/workflows/prettier.yml

+name: Prettier
+
+on:
+  pull_request:
+  push:
+    branches:
+    - master
+
+jobs:
+  prettier:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        ref: ${{ github.head_ref }}
+
+    - name: Prettify code
+      uses: creyD/prettier_action@v3.0
+      with:
+        prettier_options: --write **/*.{html,js,md}
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
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