ES Modules for the configuration file

[yhatt]

Updated cosmiconfig to the latest version, and supported ES Modules in the configuration file.

// marp.config.mjs
export default {
  allowLocalFiles: true,
  html: true,
  engine: ({ marp }) => marp.use(somePlugin)
}

Jest and pkg are not yet supported ES Modules due to the segmentation fault when running dynamic import import(). :(

• In Jest, we set up to transpile config files for testing into CommonJS manually.
• For the standalone binary powered by pkg, we are still disabling ESM import as same as Temporarily disable ESM engine resolution when using the standalone binary #525.

---

This PR is also including another improvement for the config: Export Config type to allow better experience while writing a config in IDE. @type JSDoc annotation is available in both of CommonJS and ES Modules.

/** @type {import('@marp-team/marp-cli').Config} */
const config = {
  // ...
}

export default config

If user want to swap the engine from Marp Core, Config type also can provide a suitable type for the swapped engine.

/** @type {import('@marp-team/marp-cli').Config<typeof import('@marp-team/marpit').Marpit>} */
const config = {
  engine: '@marp-team/marpit',
  options: {
    // Only suggests Marpit constructor options, not Marp Core options
  }
}

export default config