Expose nanobind's stubgen as a `py_binary` #114

Job	Run time
Lint Bazel files with buildifier	13s
Test nanobind_example on ubuntu-latest w/ Python 3.8	38s
Test nanobind_example on ubuntu-latest w/ Python 3.9	34s
Test nanobind_example on ubuntu-latest w/ Python 3.10	25s
Test nanobind_example on ubuntu-latest w/ Python 3.11	38s
Test nanobind_example on ubuntu-latest w/ Python 3.12	37s
Test nanobind_example on macos-latest w/ Python 3.8	1m 13s
Test nanobind_example on macos-latest w/ Python 3.9	1m 1s
Test nanobind_example on macos-latest w/ Python 3.10	52s
Test nanobind_example on macos-latest w/ Python 3.11	52s
Test nanobind_example on macos-latest w/ Python 3.12	1m 5s
Test nanobind_example on windows-latest w/ Python 3.8	3m 30s
Test nanobind_example on windows-latest w/ Python 3.9	1m 58s
Test nanobind_example on windows-latest w/ Python 3.10	2m 39s
Test nanobind_example on windows-latest w/ Python 3.11	2m 59s
Test nanobind_example on windows-latest w/ Python 3.12	2m 43s
	21m 57s

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