jQuery source has been migrated in gh-4541 from AMD to ES modules. To maintain
support for consumers of our AMD modules, this commits adds a task transpiling
the ES modules sources in `src/` to AMD in `amd/`.
A "Load with AMD" checkbox was also restored to the QUnit setup. Note that,
contrary to jQuery 3.x, AMD files need to be generated via `grunt amd` or
`grunt` as sources are not authored in ECMAScript modules. To achieve a similar
no-compile experience during jQuery 4.x testing, use the new "Load as modules"
checkbox which works in all supported browsers except for IE & Edge (the
legacy, EdgeHTML-based one).
Ref gh-4541
Closes gh-4554
So far, we've been testing that jQuery element iteration works with polyfilled
Symbol & transpiled for-of via a Node test with jsdom with the Symbol global
removed. Unfortunately, jsdom now requires Symbol to be present for its internal
functionality so such a test is no longer possible. Instead, it's been migrated
to an iframe test with transpiled JavaScript.
This PR also enables us to use ECMAScript 2017 or newer in Node.js code.
Closes gh-4305
