Convert eslint config to new flat config style

[mikeybinns]

I know it's still early at this point, the config standard is not stable yet, however, it would be useful for myself, and potentially others to convert the config now in a separate package (e.g. remix-eslint-flat-config), export an unstableConfig object from that package so that people who want to try out the new ESLint config style can use Remix's rules.

Eventually I would imagine that the plan is to convert the main config anyway due to the benefits of the new config system and the workarounds used in the current config so this work would need to be done at some point anyway. It's also possible you'd want to keep the old config around as a legacy config for a while, so a new package is more likely than converting the existing one.

I'm happy to work on this as a pull request, I just wanted to gauge whether this would be accepted as a PR or not before I start.

Potential up sides:

1. Less work later to convert/ create a new flat config.
2. Enables people to use the unstable config version and convert their own projects to Flat Config if they wanted to.

Potential down sides:

1. Flat config isn't final and could potentially be scrapped (I'd say this is a very low chance at this point)
2. Would add another config to update when adding new ESLint rules, or this new config would fall out of date.

[mikeybinns]

Update: I've decided to do this work anyway, work in progress in PR #4214