feat: add flat configurations

[MikeMcC399]

• closes Upgrade to new ESLint Flat configuration files #146

Issue

eslint-plugin-cypress@3.2.0 added support for the use of Flat config files with ESLint 8.57.0 and above through the compatibility utility @eslint/eslintrc.

To fully support Flat config files the plugin needs to provide corresponding configurations which do not rely on any compatibility utility.

Goals

• Offer configurations directly usable in the ESLint v9 Configuration Files default flat config format (eslint.config.*.js). The new configurations should be based on the existing .eslintrc.* configurations.
• Remove the dependency on the @eslint/eslintrc utility, also referred to as the Backwards compatibility utility
• Continue to offer configurations for the Configuration Files (Deprecated) format (.eslintrc.*)
• Prepare for removal of deprecated configurations with minimal impact for users and for plugin maintenance

Changes

• The set of flat configs is made available through eslint-plugin-cypress/flat:

Configuration	Content
configs.globals	defines globals cy, Cypress, expect, assert and chai used in Cypress test specs as well as globals.browser and globals.mocha from globals. Additionally, languageOptions of ecmaVersion: 2019 and sourceType: 'module' for backwards compatibility with earlier versions of this plugin are defined. There are no default rules enabled in this configuration.
configs.recommended	enables recommended Rules. It includes also configs.global (see above)

• Documentation is updated accordingly

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

Documentation Preview

• README
• FLAT-CONFIG
• CONTRIBUTING

[MikeMcC399]

@jennifer-shehane

• This PR is already feature-complete.

• There is however an outstanding related PR feat: add eslint api v9 compatibility ihordiachenko/eslint-plugin-chai-friendly#33. If this gets merged, then it would simplify the example using eslint-plugin-chai-friendly, so I'm currently holding the PR back and leaving it in draft status.

• There is still a lot of ballast in the repo concerning older versions which could be cleaned up in a future major release. This is however not such an urgent issue. The ESLint team still have work to do hoisting some of their own utilities into full ESLint v9 support.

[jennifer-shehane]

@MikeMcC399 Is this ready for review? It is still marked as draft.

[MikeMcC399]

@jennifer-shehane

Is this ready for review? It is still marked as draft.

I just realized that the import statements in the documentation are inconsistent, so I wanted to go back and rework to avoid that confusion. Unfortunately there is no consistency in the ESLint ecosystem about this.

[MikeMcC399]

The commits are squashed and the PR is ready for review.

[jennifer-shehane]

Forgot to hit the approve button on this one

[cypress-app-bot]

🎉 This PR is included in version 3.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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