Build: Add native ESM distribution

[Krinkle]

Source

• Convert /src/core/qunit.js to ESM export.

  This file can be used directly by ESM in theory when debugging, but in practice we'll let rollup bundle it into qunit.module.js, which is otherwise functionally the same as the source file.

• Add /src/core/qunit-commonjs.js as second entrypoint for rollup.

  Rollup would complain if attempting to export the ESM qunit.js source as "iife", as Rollup will detect the input file as having exports and insist on taking control over exporting it to a global variable. We don't want that as we control that ourselves instead.

  qunit-commonjs.js is a thin entrypoint that generates the qunit.js distribution file, which is the same as qunit.module.js except in "iife" format, with our (conditional) module.exports assignment at the end.

Build

• /qunit/qunit.js is the same ES5/CJS compatible distribution it has always been, now sourced from /src/core/qunit-commonjs.js.

• /qunit/qunit.module.js is a complete standalone distribution in ESM format. Given that IE11 doesn't support type=module, I've removed IE11 from the Babel config for this file, which makes the file a bit smaller as it uses most ES6 features natively (275K vs 250K; uncompressed, unminified).

Package

If we simply advertised the new /qunit/qunit.module.js file in package.json#exports.module, then we would break numerous Node.js and bundler use cases.

To test for these, previous commits already added /demos/bunders/, which is tested via node bin/qunit.js demos/bundlers.js, as part of npm run test-demos (and the "CI: Demos" GitHub job).

Specifically:

• A Node.js project that uses a mixture of CJS require('qunit') and ESM import 'qunit' previously shared a single logical QUnit instance since the module was just one file, and thus both used the same internal "require"-cache. This means the user's test files, or any test runners, plugins, etc, all import/require the same qunit module to define assertion plugins, add a hook listeners, or register tests. And thus everything executes correctly on the main QUnit instance.

  As-is this would break by having some of these attach stuff to a secondary instance that the main test runner (e.g. QUnit CLI) isn't running.

• A frontend project that uses a bundler or adapter (e.g. karma-qunit), and similarly involves mixed use among its test files and/or between it and any plugins, that sends the result to a browser. As-is this would end up embedding two separate copies of QUnit, unaware of each other.

To mitigate this, package.json contains two overrides:

• for node, tell ESM mode to import a Node.js-specific wrapper that re-uses and re-exports the CJS file.
• for bundler, tell CJS mode to import a bundler-specific wrapper that re-uses and re-exports the ESM file.

This is based on https://github.com/jquery/jquery/wiki/jQuery-4-exports-explainer as implemented at jquery/jquery#5429, which does the same for jQuery.

Fixes #1551.
Fixes #1724.

[mgol]

A frontend project that uses a bundler or adapter (e.g. karma-qunit), and similarly involves mixed use among its test files and/or between it and any plugins, that sends the result to a browser. As-is this would end up embedding two separate copies of QUnit, unaware of each other.

This is a bit more convoluted. Bundlers, when the module condition is used without splitting it to import vs. require conditions, both require & import in source files will use the same file - bundlers can do it as they are preprocessors, they are not concerned with some operations being async vs. sync, so they are generally fine with require('./esm-file.js') & import ... from 'commonjs-file.js'.

However, the interop of module.exports treated as a module object vs. it exposing a single API and how it corresponds to a default export in ESM is problematic and having better control over it requires us to provide separate versions for import & require. As soon as we decide that, we need to use the wrapper approach to avoid code/data duplication.

[mgol]

A few comments but nothing blocking

[matthewp]

This is amazing 🎇