Ajax: Avoid CSP errors in the script transport for async requests

Ajax: Avoid CSP errors in the script transport for async requests

Until now, the AJAX script transport only used a script tag to load scripts
for cross-domain requests or ones with `scriptAttrs` set. This commit makes
it also used for all async requests to avoid CSP errors arising from usage
of inline scripts. This also makes `jQuery.getScript` not trigger CSP errors
as it uses the AJAX script transport under the hood.

For sync requests such a change is impossible and that's what `jQuery._evalUrl`
uses. Fixing that is tracked in gh-1895.

The commit also makes other type of requests using the script tag version of the
script transport set its type to "GET", namely async scripts & ones with
`scriptAttrs` set in addition to the existing cross-domain ones.

Fixes gh-3969
Closes gh-4763