This extension introduces an alternative interface for groups of
work-items, offering several improvements over the SYCL 2020 interface:

- Shorter names for member functions, dropping the get_ prefix.

- Cleaner separation between properties of a group (e.g., a group id)
  and properties of the calling work-item (e.g., its id within a group).

- Clearer distinction between "group" concept and "work_group" class.

- New work_item class to represent a single work-item within a specific
  parent group. This class also satisfies the group concept, modeling
  a group containing a single work-item.

khr::work_item is not templated on dimensions.

The synopsis previously assumed that the extents would be one
dimensional, but it must match the ParentGroup. There is no suitable
shorthand alias or exposition-only description for this case defined
by mdspan, so replace the type with a comment.