Adds prettier plugin to eslint

[scinos]

Changes proposed in this Pull Request

• Adds prettier integration to eslint(https://prettier.io/docs/en/integrating-with-linters.html#use-eslint-to-run-prettier). This allows for a better integration with the editor, as it will display prettier errros and auto-fix them on save like eslint errors.

Testing instructions

• Configure your editor to highlight eslint errors and auto-fix them on save.
• Checkout the branch, open any .js file and type anything with formatting errors (eg: extra spaces)
• Check your editor highlights them, and fixes them on save.

[matticbot]

Test live: https://calypso.live/?branch=add/eslint-prettier-integration

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[scinos]

Using the recommended config from https://prettier.io/docs/en/integrating-with-linters.html#recommended-configuration

This does three things:

• Extends "prettier" (as before)
• Add plugin "prettier" that allow running prettier as an eslint rule
• Configure prettier rules to use "error" level.

[griffbrad]

🚢LGTM

Note for other VSCode users: the eslint extension docs explain how you'll configure codeActionsOnSave:
https://marketplace.visualstudio.com/items?itemName=dbaeumer.vscode-eslint
Typically, in settings.json, you'll just add:
"editor.codeActionsOnSave": { "source.fixAll": true }