Fixed prettier nullish chaining bug (#120)

In the `@react-native-community/eslint-config` package a quite outdated version of prettier is used which does not support the nullish chaining yet. While adding prettier to the devDependencies, this bug is fixed.

Author: maximilize

template/package.json

@@ -27,6 +27,7 @@
     "jest": "^24.9.0",
     "metro-react-native-babel-preset": "^0.56.0",
     "react-test-renderer": "16.9.0",
+    "prettier": "^1.19.1",
     "typescript": "^3.7.3"
   },
   "jest": {