feat: add ESM export

[brettz9]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Ability to import mocha from ESM in the browser #5211
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds an ESM export through a new mocha.mjs file (making the regular file to be ESM would be a breaking change). It is not entirely modular, as the base script still sets a global Mocha and mocha, but it does export mocha.

[JoshuaKGoldberg]

Thanks for sending this! Let's talk about the direction?

[JoshuaKGoldberg]

Oh 😬 I didn't think adding support for ESM would require an entirely new output file. This file is pretty huge: 600k+ characters, 620K on disk. We can't add such a large addition to package size for all users.

I had assumed the entry point would be a minimal shim, i.e.:

import "./mocha.js"

const { mocha } = globalThis;

const describe = mocha.describe.bind(mocha);
const it = mocha.it.bind(mocha);
const setup = mocha.setup.bind(mocha);

export { describe, it, setup };

Is that not doable for some reason?

[brettz9]

That makes sense. I'm unfortunately dealing with some health problems, so I think I will just nix this PR.