New eslint config

[rdimartino]

On main, running yarn lint produces some errors because of the new flat config in ESLint v9.0.0 (broken in 6e472fa)

❯ yarn lint
yarn run v1.22.22
$ eslint --cache .
(node:2502) ESLintIgnoreWarning: The ".eslintignore" file is no longer supported. Switch to using the "ignores" property in "eslint.config.js": https://eslint.org/docs/latest/use/configure/migration-guide#ignoring-files
(Use `node --trace-warnings ...` to show where the warning was created)

Oops! Something went wrong! :(

ESLint: 9.1.0

Error: Could not find config file.
...

This PR updates to the new ESLint flat config and also fixes one error from successfully running yarn lint