Fresh RN projects fails ESLint / Prettier by default (#25478)

Summary:
Fixes #25477

This PR adds a `.prettierrc.js` config file to the HelloWorld template.

`eslint-config-react-native-community` includes custom settings for some rules which conflict with Prettier's default settings.

Consequently, if you run `eslint` immediately after scaffolding a new app you get errors (see linked issue).

This PR configures Prettier to be compatible with both the existing ESLint config and the existing project template (with no code changes to the latter).

## Changelog

[General] [Changed] - Added a default Prettier config for new projects
Pull Request resolved: #25478

Test Plan:
- The following screenshots show the output of `yarn lint` before and after these changes
- These were run immediate after running `npx react-native-cli init RN060`

### Without these changes

- Linting errors on new projects
- Unfixable automatically due to conflicting rules
<img width="1116" alt="Screenshot 2019-07-03 at 17 44 55" src="https://user-images.githubusercontent.com/2393035/60610078-f6b44d00-9dba-11e9-826f-1524b949e4fd.png">
<img width="1116" alt="Screenshot 2019-07-03 at 17 45 07" src="https://user-images.githubusercontent.com/2393035/60610085-fb790100-9dba-11e9-9a9c-33f4cfefd51e.png">

### With these changes

- Brand new projects will not produce lint errors out of the box
<img width="1116" alt="Screenshot 2019-07-03 at 17 48 25" src="https://user-images.githubusercontent.com/2393035/60610266-57dc2080-9dbb-11e9-8a55-fd09f3549c17.png">

Differential Revision: D16223094

Pulled By: cpojer

fbshipit-source-id: bd2c00b1fcf27b1afcad8c18b357b95a3900bdf7

template/_prettierrc.js

@@ -0,0 +1,6 @@
+module.exports = {
+  bracketSpacing: false,
+  jsxBracketSameLine: true,
+  singleQuote: true,
+  trailingComma: 'all',
+};