Refactor mock custom formatter to rename to CJS

[ybiquitous]

Which issue, if any, is this issue related to?

Ref #5291

Is there anything in the PR that needs further explanation?

Note that a custom formatter in ESM is unsupported.

[Mouvedia]

Note that a custom formatter in ESM is unsupported.

stylelint/lib/__tests__/standalone-formatter.test.mjs

Line 5 in 6d040b2

import stringFormatter from '../formatters/stringFormatter.mjs';

I guess it's different for formatter.

[ybiquitous]

@Mouvedia We're currently using require(). But we should support ESM as well.

stylelint/lib/cli.mjs

Lines 396 to 400 in 958bf36

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

[ybiquitous]

I've just opened an issue (#7267) for an ESM custom formatter.

[Mouvedia]

LGTM

offtopic
https://github.com/stylelint/stylelint/blob/v16/lib/__tests__/fixtures/custom-formatter.mjs
should probably be moved in the formatters directory

[ybiquitous]

Thanks for the review.

offtopic
v16/lib/tests/fixtures/custom-formatter.mjs
should probably be moved in the formatters directory

This file is used only by resolveCustomFormatter.test.js:

stylelint/lib/__tests__/resolveCustomFormatter.test.mjs

Line 8 in ca2109c

const aRelativePath = 'lib/__tests__/fixtures/custom-formatter.mjs';