[Spec change] Add sycl_khr_free_function_commands extension

Please review whether https://github.com/KhronosGroup/SYCL-Docs/pull/921 by @slawekptak requires any changes to the CTS.

**If changes are required**: Open a new PR addressing the changes and link it to this issue.
**If no changes are required**: Close this issue and proceed with the spec PR.