Split configs

[DanWebb]

Separate configs for

• Next (eslint-config-next + eslint-plugin-jsx-a11y) note we don't need eslint-plugin-react because its already included
• React (eslint-plugin-jsx-a11y + eslint-plugin-react)
• Everything else

[DanWebb]

With eslints new flat file config we can write a function that exposes settings like { react: true } to enable different configs like this example