* Update package.json

* Update `next` example dependencies

* Update `parcel` example dependencies

* Update `razzle` example dependencies

requestImageSize was removed based on this commit
8e1b5df

* Implement ESLint

- integrate Prettier as an ESLint plugin
- run Prettier across the codebase
- set up crucial import rules to avoid issues like #296
- enforce ESLint through a pre-commit hook, enforcing Prettier as well!
- run ESLint on Travis CI

* Fix ESLint errors

* Bump ESLint

* Tell lint-staged to autofix staged hunks

lint-staged supports partially staged files, so it's safe to use
ESLint's autofix feature to fix and re-stage staged hunks to make
working even more seamless.

* Match ESLint & Prettier configuration with unified

https://github.com/unifiedjs/design-system/blob/b34a4bb03d85fc79be885a996e1d769247d8e1d3/package.json

* Add format script which autofixes all JS files

This is useful when changing ESLint configuration.

* Fix ESLint errors

Also remove unnecessary rules.

* Run ESLint as part of the "test" script

* Enforce markdown and prose code style

Overview:

* Add configuration for markdown (excluding the test files)
* Add `@mdx-js/remark-mdx` to parse imports, exports, and JSX to MDAST,
  and stringify them back to markdown. It’s private, and not tested yet,
  but it makes sure remark can run on mdx files too (note: we need to talk
  about the license file there)
* Add `remark` to the `"format"` script, fixing code where possible
* Add `remark` to the `"test"` script, but not outputting in this case
* Add to lint-staged

Documentation:

* Use reference links and definitions everywhere
* Use `*` for lists and strong; add more spaces to lists
* Fix broken links, references, and remove superfluous definitions
* Fix AST spec (add comment type, default field)
* Add missing code languages
* Wrap lines around 80 chars
* Use smart quotes (`“”` and `‘’`)

* Add lint-staged for markdown

* Update package.json

Co-Authored-By: wooorm <tituswormer@gmail.com>

* Update ast.md

* Update `remark`, other dependencies

* Remove superfluous regex

* Downgrade `styled-components`

* Fix `lint-staged` for markdown

* Add `changelog.md`

* Improve npm search rank by having a changelog
* Fix humans looking for where changes are stored

* Add `code-of-conduct.md`

* Update `package.json`s

* Update `readme.md`s

* Update core `readme.md`

* Add note about financial support to `contributing.md`

* Fix Tim’s email

* Stabilize UserLayout

Fixes #307

Pulls the layout for an MDX file into a reference that will be stable
across renders. Previously, a layout defined in MDX content as an
inline arrow function:

```js
export default ({ children, ...props }) => <div {...props}>{children}</div>
```

would result in function defined inline in render, causing the
reference to be created each time resulting in a remounting.

This PR stablizes the reference by placing the layout outside of
render in a class property, resulting in no remount.