Support dictionary-style CF indexing

[Kkkakania]

Summary

• support position-based indexing such as obj.cf[{"T": 0}] for both DataArray and Dataset objects
• add a .cf.loc wrapper that translates CF names for label-based reads and assignments
• reuse the existing isel/sel mapper so one CF axis can expand to multiple concrete dimensions consistently

Closes #363.

Tests

• added coverage for DataArray and Dataset reads, .loc assignment, dask-backed inputs, and one-to-many axis mappings
• uv run --frozen --group test pytest cf_xarray/tests -q (296 passed, 1 skipped)
• uvx pre-commit run --all-files