[TASK] Use extension source paths in icon API

Also introduce internal PathUtility API to resolve
the web path of extension resources and a helper
public API method to check whether a path is an
extension resource path.

Start using the new API throughout the core.
All checks of EXT prefix are replaced,
further usages of getAbsoluteWebPath will
be replaced in subsequent patches.

Releases: master
Resolves: #95022

Change-Id: I27994f309af8b35ad3822a5be59d81d164fa9cd0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70805


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>