Adds lint rules to the Next.js ESLint plugin to:
- Disallow importing `next/head` inside `pages/_document.js`
- Disallow importing `next/document` outside of `pages/_document.js`
Both rules will be surfaced as **errors** within the recommended config of the plugin.
Fixes#13712#13958
Adds a lint rule warning to the Next.js ESLint plugin if a custom Google Font is added at page-level instead of with a custom document (`.document.js`)
_Note: This will be generalized to include more font providers in the near future._
This PR re-includes ESLint with some notable changes, namely a guided setup similar to how TypeScript is instantiated in a Next.js application.
To add ESLint to a project, developers will have to create an `.eslintrc` file in the root of their project or add an empty `eslintConfig` object to their `package.json` file.
```js
touch .eslintrc
```
Then running `next build` will show instructions to install the required packages needed:
<img width="862" alt="Screen Shot 2021-04-19 at 7 38 27 PM" src="https://user-images.githubusercontent.com/12476932/115316182-dfd51b00-a146-11eb-830c-90bad20ed151.png">
Once installed and `next build` is run again, `.eslintrc` will be automatically configured to include the default config:
```json
{
"extends": "next"
}
```
In addition to this change:
- The feature is now under the experimental flag and requires opt-in. After testing and feedback, it will be switched to the top-level namespace and turned on by default.
- A new ESLint shareable configuration package is included that can be extended in any application with `{ extends: 'next' }`
- This default config extends recommended rule sets from [`eslint-plugin-react`](https://www.npmjs.com/package/eslint-plugin-react), [`eslint-plugin-react-hooks`](https://www.npmjs.com/package/eslint-plugin-react-hooks), and [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next)
- All rules in [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next) have been modified to include actionable links that show more information to help resolve each issue
- Introduces a lint rule which points towards the unwanted polyfill.io features.
- Aim here is to make the user aware that which of the requested features are actually required vs which are already covered under `next-polyfills`
Next step: If the remaining required polyfills amounts to a only a few KBs then its better to include them in 1P javascript and remove the third party render blocking script tag.
