chore: migrate to ESLint Flat Config

[qnighy]

Background:

• https://eslint.org/blog/2022/08/new-config-system-part-1/
• https://eslint.org/docs/latest/use/configure/configuration-files-new

Also it packs all the configs into the project-wide one.

TODO: allow running ESLint from the root package

[changeset-bot[bot]]

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Latest commit: 6fad600d72155f5b6937fa2631c2b3e411b1b5c4

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