Add support for eslint flat config format

[rhysd]

Fix #341

This PR adds support for the eslint flat config. Now eslint-plugin-mocha can be used in eslint.config.js.

import eslint from '@eslint/js';
import mocha from 'eslint-plugin-mocha';

export default [
    eslint.configs.recommended,
    mocha.configs.flat.recommended
];

Changes are as follows:

• Since the configuration format is different from legacy eslintrc config, new objects for all and recommended presets were added to mochaPlugin.configs.flat separately.
• globals package was added as dependency because env config was dropped at flat config. Globals such as describe need to be added in languageOptions and globals package provides the list of mocha's globals.
• Instruction for flat config was added to README.md document