[TASK] Auto-register set labels from labels.xlf

The set language files introduced in #104831
are now auto-loaded from labels.xlf within
the sets folder, if the file is provided
and the set configuration does not reference
an explicit labels xlf file.

TYPO3 documentation is adapted to understand this change via:
https://github.com/TYPO3-Documentation/render-guides/pull/737

Releases: main
Resolves: #104840
Related: #104831
Change-Id: If00ae72da97b65768d925de48f7850e80bc44b3b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85907


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>