Ajax: Support `headers` for script transport even when cross-domain

Ajax: Support `headers` for script transport even when cross-domain

The AJAX script transport has two versions: XHR + `jQuery.globalEval` or
appending a script tag (note that `jQuery.globalEval` also appends a
script tag now, but inline). The former cannot support the `headers`
option which has so far not been taken into account.

For jQuery 3.x, the main consequence was the option not being respected
for cross-domain requests. Since in 4.x we use the latter way more
often, the option was being ignored in more cases.

The transport now checks whether the `headers` option is specified and
uses the XHR way unless `scriptAttrs` are specified as well.

Fixes gh-5142
Closes gh-5193