Feature: #83476 - Load merged JS files asynchronous

Description

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.

Example:

config.concatenateJs = 1

page = PAGE
page.includeJSFooter {
    test = fileadmin/user_upload/test.js
    test.async = 1

    test2 = fileadmin/user_upload/test2.js
    test2.async = 1
}