SYCL_KHR_GROUP_INTERFACE prototype

[PeterTh]

This is a prototype implementation of the SYCL_KHR_GROUP_INTERFACE extension. https://github.com/KhronosGroup/SYCL-Docs/pull/638

The C++23-exclusive features are not implemented as of yet, but the rest should be feature complete.

Tests are adapted from the CTS.