feature: Support Flat configs in v8

[lotmek]

Description

• Handle flat config in rule-finder.js
• Add a new --flatConfig flag
• Load ESLint class using FlatESLint class
• Add unit tests for basic flat config

[ljharb]

LGTM, this will be semver-minor

[ljharb]

@lotmek it looks like the exception is always being thrown.

[lotmek]

@lotmek it looks like the exception is always being thrown.

Hello @ljharb, tell me if I'm wrong, but I think flat configs are not supported in ESLint v7

I can add a condition to skip the flat config tests if ESLint version is lower than 8, what do you think of it ?

  if (semver.satisfies(eslintPkg.version, '>= 8')) {
    describe('flat config', () => {
        ...
    })
  }

[ljharb]

I have eslint 8.56 installed and it's still failing locally, fwiw.

Certainly the flat config tests should be ensuring it throws in eslint 7, good catch.

[lotmek]

I have eslint 8.56 installed and it's still failing locally, fwiw.

Certainly the flat config tests should be ensuring it throws in eslint 7, good catch.

I did some changes, can you try again ?

[ljharb]

LGTM, thanks