[FEATURE] Load merged JS files asynchronous

The async attribute is now assigned to the script tag of the concatenated
JS files if all files have the async attribute enabled in TypoScript.

Resolves: #83476
Releases: master
Change-Id: If4d5f03cac5920cf0bcccefb2e91cc229f9b9e77
Reviewed-on: https://review.typo3.org/57130


Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>