Expose LocalBackend from top-level package

[NenoL2001]

Summary

• expose art.LocalBackend via lazy top-level export
• keep backend-only dependencies optional during import art
• add a regression test for the top-level export

Notes

• no runtime behavior change beyond restoring the documented top-level API

[Kovbo]

Should we instead standardize on using from art.local import LocalBackend and remove art.LocalBackend from public API? I think it matches our examples.
PR
cc @bradhilton