[BUGFIX] IRRE: Rewrite stylesheet loading sequence

The stylesheet loading process in IRRE was kept for several years and
moved from version to version. A recent change in #103375 migrated a
usage of `delete` to `.splice()`, breaking stylesheet loading as the
original iterated object was modified.

Since the code was quite dated and had other potental bugs, the whole
loading sequence has been transformed to functional programming style
to reduce complicated, potentially error-prone, array operations.

Resolves: #103971
Related: #103375
Releases: main, 12.4
Change-Id: I18caafc68ceebce53723b7208270043faafc7458
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84487


Tested-by: rogier Helmer <rogier.helmer@maxserv.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: rogier Helmer <rogier.helmer@maxserv.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>