ESLint flat config support

[mrmckeb]

We plan to implement support for ESLint's new flat config format around the time that ESLint v9.0.0 launches. At that time, this will be the default config format for ESLint.

Until that time, anyone moving to v9.0.0 (or a pre-release) can set the ESLINT_USE_FLAT_CONFIG environment variable to false as a workaround.

The move to the flat config will be a breaking change in this package.

See this post for more: https://eslint.org/blog/2023/10/flat-config-rollout-plans/

[mwskwong]

Just wanna add that ESLint v9 has hit stable, with flat config being the default. It may be worth focusing on moving to flat config now