Add prettier as a dependency (#487)

package.json

@@ -45,6 +45,7 @@
     "inquirer": "^6.2.1",
     "jest-snapshot": "^23.6.0",
     "mocha": "^5.2.0",
+    "prettier": "2.2.1",
     "puppeteer": "^1.11.0",
     "rollup": "^2.3.3",
     "rollup-plugin-commonjs": "^9.2.0",

yarn.lock

@@ -2489,6 +2489,11 @@ preserve@^0.2.0:
   resolved "https://registry.yarnpkg.com/preserve/-/preserve-0.2.0.tgz#815ed1f6ebc65926f865b310c0713bcb3315ce4b"
   integrity sha1-gV7R9uvGWSb4ZbMQwHE7yzMVzks=
 
+prettier@2.2.1:
+  version "2.2.1"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-2.2.1.tgz#795a1a78dd52f073da0cd42b21f9c91381923ff5"
+  integrity sha512-PqyhM2yCjg/oKkFPtTGUojv7gnZAoG80ttl45O6x2Ug/rMJw4wcc9k6aaf2hibP7BGVCCM33gZoGjyvt9mm16Q==
+
 pretty-format@^23.6.0:
   version "23.6.0"
   resolved "https://registry.yarnpkg.com/pretty-format/-/pretty-format-23.6.0.tgz#5eaac8eeb6b33b987b7fe6097ea6a8a146ab5760"