benchmark

Bump nanobind-bazel to v2.1.0, add stubgen target (#1824)

Adds a stub file for the `google_benchmark._benchmark` submodule,
generated with the new `nanobind_stubgen` rule released in nanobind_bazel
v2.1.0.

Tweaks the setup.py logic a little bit to package stub files with the
rest of the build artifacts. Also explicitly adds the generated stub and
marker files to the list of package data artifacts.

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>