Update to flat eslint config

[nickofthyme]

In eslint@v9 they removed the deprecated configuration file, which we use, in favor of the new flat configuration file. We can opt-in to use the old format but that also creates errors due to other underlying changes, so best to just migrate to the new flat config.

This is more involved than just a few tweaks as they changed the entire paradigm of the old config where instead of overrides we now list multiple configurations in order of importance. See full migration guide here, but the biggest changes are...

• Migrate from overrides to multiple configs.
• Changes import flow of plugins. Some of which may not support new flat format requiring a FlatCompat utility.
• Changes extending configurations. Especially related to typescript-eslint.
• Changes .eslintignore functionality
• Migrate to use https://eslint.style/guide/getting-started