[TASK] Add type declarations for GU::fixWindowsFilePath

Also add casts to callers that are not type-safe (yet).

Also convert one call to use a first-class callable to improve
static analysis.

Resolves: #100686
Releases: main
Change-Id: I0193a10238b9296c570268355480316e8ddfa4db
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78772


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>