[TASK] Add composer-mode to our acceptance test matrix

All applicable acceptance tests are now also executed
in composer mode. Tests that check classic-mode specific
functions are excluded and are therefore now tagged
as classic-mode test.

The composer mode instance is generated with our
CLI setup tools, allowing to mimic the realworld
case where a instance is creating via our setup tools
instead of from fixtures (which we still use for classic
mode tests, but try to reducde/avoid for composer mode tests).

The existing existing classic-mode test acceptance test
execution will keep running as-is.

The v12 backport additionally imports pieces from #102440
as t3editor extension is enabled in this composer-mode,
while v12 classic mode is tested (for historic reasons)
without t3editor extension enabled.

Commands executed:

  > composer require --dev "typo3/cms-styleguide":"^12.0.5"

Resolves: #103297
Releases: main, 12.4, 11.5
Change-Id: I64973f110931b51ed2ef7ef8f8cc3411834fcf37
Reviewed-on: https://review....