[FEA]: Debugger pretty-printers: `cuda::mr::memory_resource`

[Jacobfaib]

Create a lldb and gdb pretty printer for cuda::mr::memory_resource. This should extend the existing one which is rather bare bones.

The printer should live under libcudacxx/share/libcudacxx/<debugger>/memory_resource.py for each debugger.

In addition, the pretty printers should be exhaustively tested, with tests living under libcudacxx/test/debugging/memory_resource. See the existing tests in that directory for inspiration.