Update to ESLint 9 when it comes out

[DylanSp]

https://eslint.org/blog/2023/11/whats-coming-in-eslint-9.0.0/

The config changes probably have the biggest effect. Requiring Node 20 would be good to note in the README, as well.

Once it gets released, make sure typescript-eslint works with ESLint 9 (may require waiting some more); that might be a good time to implement #31 (updating typescript-eslint's config to the currently recommended style) as well.