Add support for stylelint v16

[paulfalgout]

In v16 the formatters were changed to Promises
https://github.com/stylelint/stylelint/blob/main/docs/migration-guide/to-16.md#changed-nodejs-api-stylelintformatters-object

So the defaults no longer work.

[cascornelissen]

Thanks for reporting this. My initial reaction is that this should be a simple (hopefully backwards-compatible) change where we'd need some additional async/await keywords in the following part.

stylelint-bare-webpack-plugin/lib/index.js

Lines 28 to 34 in 0bdd75b

	formatter: (() => {
	if ( get(stylelint, 'formatters.string') ) {
	return stylelint.formatters.string;
	} else if ( moduleAvailable('stylelint/dist/formatters/stringFormatter') ) {
	return require('stylelint/dist/formatters/stringFormatter').default;
	}
	})()

I'm open to a PR if anyone wants to try this out. As mentioned I'd love to support this in a backwards-compatible way so let's ensure it works with v16 but also with earlier versions so that we can maintain the current level of flexibility.