Rework linting config

[Willem-Jaap]

This PR reworks the eslint & prettier configuration

Changes

Removed yarn.lock, there is already a package-lock.json file. In my opinion we should stick to a single package lock file, in previous pr's it has been mentioned yarn is not used.

Removed babel-eslint from the devDependencies, it is unmaintained and replaced by the already present @babel/eslint-parser. (Read about it here: https://www.npmjs.com/package/babel-eslint), this migration was already done in the eslint config (#28).

Removed eslint-plugin-prettier from the devDependencies, the prettier and eslint teams recommend to use each tool separately. (https://prettier.io/docs/en/integrating-with-linters.html#eslint).
A linter should be used for linting, a formatter for formatting. It also wasn't reporting any prettier errors to eslint. This behaviour is implemented by extending eslint-config-prettier.

Removed eslint-plugin-next from the devDependencies, @next/eslint-plugin-next, the plugin that powers the config is already present. Because we use a custom eslint config we implement the rules ourselves by extending that core plugin.

Implemented the next recommended rules in the custom eslint config. This now correctly extends the next config and reports the correct errors.

I also removed some unused code reported by eslint.

package.json is not formatted according to the prettier rules, along with a lot of the files in the repo. I'd love to run prettier --write . but thats a maintainer decision 😅

[taylorotwell]

Why is open removed from the Dropdown if it is used just a few lines later?

[Willem-Jaap]

The open state on line 34 is not derived from this useState hook.
This is a render prop, also called the function as child pattern (https://reactpatterns.js.org/docs/function-as-child-component/)

Instead of rendering a component headless ui returns the state as property in a callback so you can render custom components yourself: https://headlessui.com/react/menu#using-render-props

[taylorotwell]

Thanks 👍