Migrate ESLint config to flat config

[targos]

Refs:

• https://eslint.org/docs/latest/use/configure/migration-guide
• https://eslint.org/blog/2022/08/new-config-system-part-2/
• https://eslint.org/docs/latest/use/configure/configuration-files

I started the work. I'm just opening an issue to track it.