Support custom formatter for ESM

[ybiquitous]

What is the problem you're trying to solve?

Currently, we support only a custom formatter written in CommonJS (CJS). But, it would be useful if people could also specify a custom formatter in ESM.

stylelint/lib/cli.mjs

Lines 396 to 400 in 958bf36

	if (cli.flags.customFormatter) {
	const customFormatter = resolveCustomFormatter(cli.flags.customFormatter);
	formatter = require(customFormatter);
	}

Related to #6930

What solution would you like to see?

I suggest supporting a custom formatter written in ESM. This means we need to change the current requiring logic.

[mattxwang]

This seems like a reasonable idea to me! From my understanding, it wouldn't be too complicated right (?) - as we can synchronously load CJS from ESM code?

(perhaps we could also look to the community and see which formatters are popular, which could help us guide the API)

[ybiquitous]

I guess we could use dynamic import (aka import()), which is available both ESM and CJS modules as the Node.js document says:

Dynamic import() is supported in both CommonJS and ES modules. In CommonJS modules it can be used to load ES modules.

We must change the code using Promise when adopting this approach since import() returns a Promise. But this should not be problematic because the cli.mjs's main function is already an async function.