Add a proposed extension specification for `work_group_memory`, a
lighter weight API to allocate device local memory for an nd-range
kernel.

Add a list of restrictions that, when followed, provide a guarantee
that a kernel written in the free-function kernel syntax can be
launched directly via Level Zero or OpenCL.

Add example using `work_group_memory` in free function kernel.

The core SYCL spec requires an `errc::kernel_argument` exception
if a `local_memory` object is passed to a single-task or simple "range"
kernel.  Add the same requirement for `work_group_memory`.