[FEATURE] API for AJAX requests

In order to become independent of jQuery, a new API to perform AJAX
requests is introduced. This API implements the fetch API available
in all modern browsers.

Resolves: #89738
Releases: master
Change-Id: Ibdc1a7047b29402dea21b6dd7a48217b9afb849e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62372


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>