This commit cleans up a few comments & configurations that are out of date
after the migration to ES modules backed by a Rollup-based compilation.
Also, de-indent AMD modules. This will preserve a more similar
structure to the one on 3.x-stable where the body of the main `define`
wrapper is not indented.
Closes gh-4705
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
