Switch to flat config

[fregante]

I'm opening this tracking issue to collect related tickets.

This change blocks:

• https://github.com/sindresorhus/eslint-plugin-unicorn/issues/2151
• https://github.com/sindresorhus/eslint-plugin-unicorn/issues/2163

[silverwind]

Does it have to be a "switch"? There are a number of plugins that support both configs at the same time and I think it would ease the migration.

[fregante]

I think the flat config is already supported:

• https://github.com/sindresorhus/eslint-plugin-unicorn/pull/1886

So this issue is to drop support for the old config, essentially. It's not something that will happen anytime soon, I suppose.