feat(starters): Add .prettierrc to starters (#11786)

* feature(prettier): Add .prettierrc to starters

* Add format script and prettier and prettier as a dev dependency

* Move format script up after develop

* feature(prettier): Add .prettierrc to starters

* Add format script and prettier and prettier as a dev dependency

* Move format script up after develop

* Add .prettierrc to filetree on README files of all starters

* Remove duplication of devDependencies on hello-world starter

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "build": "gatsby build",
     "develop": "gatsby develop",
+    "format": "prettier --write src/**/*.{js,jsx}",
     "start": "npm run develop",
     "serve": "gatsby serve",
     "test": "echo \"Write tests! -> https://gatsby.app/unit-testing\""
@@ -16,6 +17,9 @@
     "react": "^16.8.3",
     "react-dom": "^16.8.3"
   },
+  "devDependencies": {
+    "prettier": "^1.16.4"
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/gatsbyjs/gatsby-starter-hello-world"