[!!!][FEATURE] Override any backend template with TsConfig

This patch finishes the migration to the new backend
templating strategy. Affected are the backend page
module and the dashboard extension:

* Bigger refactoring of PageLayoutController
  towards more modern and streamlined code with
  much less class state.
* Transition of fillDefaultsByPackageName() in
  the page module for the "grid" layout towards
  TsConfig based template overrides.
* Refactoring of the dashboard main controller
  to avoid fillDefaultsByPackageName().

The new template override API based on TsConfig
is now documented with a feature ReST file. The
new template override strategy is breaking for
existing overrides based on FE TypoScript and
documented with a breaking ReST file including
a detailed migration path. The ext:dashboard
documentation is adapted accordingly.

As a result, all backend controllers that do not
rely on extbase no longer parse FE TypoScript.
This is especially useful for the central page
module: Clicking around on a v12 bootstrap_package
page tree in the page module without the patch leads
to around 80 to 120 milliseconds parsetime on my
machine - depending on the number and complexity
of content elements and the cache state. This
systematically shrinks to around 50 to 70 milliseconds
with the patch: The system no longer loads tons of
extbase related classes, dependency injection is more
effective, and the entire FE TypoScript parsing is gone.
With cold caches ("Flush all caches" backend button),
the difference is around 100 milliseconds for the
first call on my system - in general, the bigger the
instance, the higher the improvement.

Change-Id: Ia244495db592526633d01e7f504502e297bd2ef9
Resolves: #96812
Related: #96730
Related: #96614
Related: #90348
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73366


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