Initial migration to new ESLint flat config format

[emmenko]

Note

This is an exploratory branch to attempt the migration of the ESLint configs to the new ESLint flat config format.
This is not ready for review yet.

ESLint has been working on a major refactoring of their config format which they call "flat" (or simply "new config").

I believe this will be the default format from version 9 onwards, however we can already opt into it.

I also found this article doing a good job in explaining the differences and how you can migrate the config piece-by-piece.

If someone has time and wants to explore this, let's use this branch.

• Migrate repository ESLint config
• Migrate eslint-config-mc-app (will it be a breaking change or can we be backwards compatible?)

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[ByronDWall]

@emmenko you are correct that eslint v9+ requires this configuration format. Updating any app that uses '@commercetools-frontend/eslint-config-mc-app' to eslint v9 results in errors such as these:

Test suite failed to run
  
      Invalid Options:
      - Unknown options: extensions, ignorePath, useEslintrc, reportUnusedDisableDirectives, resolvePluginsRelativeTo, rulePaths
      - 'extensions' has been removed.
      - 'resolvePluginsRelativeTo' has been removed.
      - 'ignorePath' has been removed.
      - 'rulePaths' has been removed. Please define your rules using plugins.
      - 'reportUnusedDisableDirectives' has been removed. Please use the 'overrideConfig.linterOptions.reportUnusedDisableDirectives' option instead.
  
        at processOptions (node_modules/eslint/lib/eslint/eslint-helpers.js:843:15)
        at new ESLint (node_modules/eslint/lib/eslint/eslint.js:449:34)

I'm assuming that updating the eslint config isn't something that is planned in the immediate future?