Build: Auto-convert sources to AMD

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

.gitignore

@@ -16,6 +16,8 @@ npm-debug.log*
 /dist/*
 !/dist/.eslintrc.json
 
+/amd
+
 /node_modules
 
 /test/data/core/jquery-iterability-transpiled.js