Support ESLint flat config

[bmish]

This is targeting a v11 branch I created for backporting any changes to v11 (although note that I haven't implemented this for the WIP v12 release yet). I merged this branch into v12 in this PR and updated the new v12 configs to support flat config.

---

This publicly supports the new ESLint flat config format by adding a new plugin export that can be used like this:

// eslint.config.js
const eslintPluginEmberRecommended = require("eslint-plugin-ember/configs/recommended");

module.exports = [
    ...eslintPluginEmberRecommended,
];

Using legacy config is unchanged:

// .eslintrc.js
module.exports = {
  extends: ['plugin:ember/recommended']
};

In a future breaking change / major version when flat config is more pervasive, we can drop support for the legacy config.

Prior art / related:

• feat: support ESLint flat config platinumazure/eslint-plugin-qunit#443
• feat: support eslint.config.js eslint-community/eslint-plugin-n#95
• feat: support eslint.config.js eslint-community/eslint-plugin-eslint-plugin#347
• Switch to ESLint flat config internally #2018
• https://eslint.org/docs/latest/use/configure/migration-guide