Bump nanobind-bazel to v2.1.0, add stubgen target

[nicholasjng]

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.

[nicholasjng]

Side note: The generated stub causes the following mypy errors:

1. Unreachable overloads on Counter.Flags.__init__()
2. Too narrow typing of arg in Counter.Flags.__or__() (expects int, got IntEnum)
3. Too narrow typing of arg in UserCounters.__eq__() and UserCounters.__ne__() (expects object, got UserCounters).

These all seem to be problems in nanobind.stubgen itself, so there is not a lot to do here except maybe contribute fixes or use a pattern file to rewrite the stub directly at build time.