[TASK] Use Locale object in Site Configuration

This change introduces the combination of the newly
added Locale class and SiteLanguage, allowing to work
more with the objects than with strings as before.

We're also adding an internal "POSIX" formatted logic
to the Locale object, so the setLocale() calls can still
work as before.

This patch basically provides a bridge between
the previously used POSIX format ("de_AT.utf-8")
and the BCP-47 definition of language tags which is
the base for the Locale class. However, both formats
can now be used (in site configuration), and are properly
formatted in setLocale() or in HTTP response headers
in Frontend, which was a limitation before.

Resolves: #99812
Releases: main
Change-Id: Iee62e537f7dd334d614e4922cb7a0dd7222c379d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77685


Tested-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>