package: Add prettier configuration and command

fix #1 (closed)

{{ cookiecutter.project_slug }}/package.json

@@ -31,6 +31,7 @@
     "file-loader": "^6.2.0",
     "html-webpack-plugin": "^5.5.0",
     "mini-css-extract-plugin": "^2.6.1",
+    "prettier": "^2.7.1",
     "react-dev-utils": "^12.0.1",
     "style-loader": "^3.3.1",
     "ts-loader": "^9.3.1",
@@ -43,7 +44,8 @@
   "scripts": {
     "start": "webpack serve --config webpack/webpack.config.js --mode development",
     "build": "webpack --config webpack/webpack.config.js --mode production",
-    "lint": "eslint ."
+    "lint": "eslint .",
+    "format": "prettier src"
   },
   "eslintConfig": {
     "extends": [