[FEA]: Debugger pretty-printers: `cuda::std::expected`

Create a `lldb` and `gdb` pretty printer for `cuda::std::expected`.

The printer should live under `libcudacxx/share/libcudacxx//expected.py` for each debugger.

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

Start by reading the existing tests under libcudacxx/test/debugging/expected and inspect the debugger-specific printer patterns in libcudacxx/share/libcudacxx//. Add expected.py for both lldb and gdb, then extend the expected tests exhaustively so they cover the displayed value and states for cuda::std::expected.

Written by the indexing model from the issue text.