[TASK] Build submodules from scheduler module parts

This change splits up
* Edit/List/Add view of Scheduler
* Check Setup
* Available Tasks

in each a separate controller + a custom third-level
module.

This way, it is then possible to separate the
main scheduler module into custom actions (via subroutes)
making the code a tiny bit more manageable.

The previous "system_txschedulerM1" module name is now
aliased to "scheduler_manage" (third-level), as the main
scheduler module is now solely called "scheduler". The old
name is kept for backwards-compatibility reasons.

A note for historical fans: "M1" stands for "module 1" within
the "tx_scheduler" extension.

Resolves: #100104
Releases: main
Change-Id: I2ba7c76899853864de42a11f2dc45c40ce699c91
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78038


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@bu...>