Update ESLint config to use flat config system

[eventualbuddha]

ESLint changed their configuration system from e.g. .eslintrc.json & .eslintignore to so-called "flat" config in eslint.config.js. The entire project can (and probably should) use a single eslint.config.js file at the root. This change appears to be part of a change to use ES modules to import/export config rather than using strings that magically tell ESLint how to load the code. This is a non-trivial change.

https://eslint.org/blog/2022/08/new-config-system-part-2/