This is a partial refactoring of ext:lowlevel. There
are some loose ends like HTML in the searchAction()
and some other details. Those might be target of a
bigger refactoring. However, the patch cleans up
controller and view and improves separation of
concerns already:

* Rename mainAction() to handleRequest()
* Use BackendTemplateView
* Hand over template to render as view->render('MyTemplate')
* Use DI in related controllers
* Switch from a ServiceProvider.php to casual Services.yaml
* Declare strict_types=1
* Return and property types in PHP classes
* PageRenderer JS module loading via fluid, not PHP
* f:translate with full LLL: keys

Change-Id: I0afe0938990f145e77aedd131f366226142382f3
Resolves: #96530
Related: #96513
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73007


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>