Add eslint alongside prettier

[martinpengellyphillips]

Useful for collaboration (shared rule definition) and automating some good practices.

See https://prettier.io/docs/en/integrating-with-linters.html for how to avoid conflicts with prettier.

For example, consider turning on this rule following discussion on https://github.com/thisbeyond/solid-dnd/pull/19

And https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/order.md and maybe https://eslint.org/docs/rules/padding-line-between-statements