Add nb::is_flag() annotation to Counter::Flags

[nicholasjng]

This saves us the definition of __or__, because we can just use the one from enum.IntFlag.

cc @hawkinsp for a review, since you mentioned this should be possible after flag enum support is added to nanobind, which it is now, starting in v2.2.0.

I did have to use both nb::is_arithmetic() and nb::is_flag() due to combination issues, but it seems the C++ side is aware of this:

benchmark/include/benchmark/benchmark.h

Lines 720 to 726 in 4e3f2d8

	// A helper for user code to create unforeseen combinations of Flags, without
	// having to do this cast manually each time, or providing this operator.
	Counter::Flags inline operator|(const Counter::Flags& LHS,
	const Counter::Flags& RHS) {
	return static_cast<Counter::Flags>(static_cast<int>(LHS) |
	static_cast<int>(RHS));
	}

[nicholasjng]

For completeness, the traceback when just using nb::is_flag():

Traceback (most recent call last):
  File "/Users/nicholasjunge/Workspaces/c++/benchmark/bindings/python/google_benchmark/example.py", line 140, in <module>
    benchmark.main()
    ~~~~~~~~~~~~~~^^
  File "/Users/nicholasjunge/Workspaces/c++/benchmark/.venv/lib/python3.13/site-packages/google_benchmark/__init__.py", line 136, in main
    return app.run(_run_benchmarks, argv=argv, flags_parser=_flags_parser)
           ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nicholasjunge/Workspaces/c++/benchmark/.venv/lib/python3.13/site-packages/absl/app.py", line 308, in run
    _run_main(main, args)
    ~~~~~~~~~^^^^^^^^^^^^
  File "/Users/nicholasjunge/Workspaces/c++/benchmark/.venv/lib/python3.13/site-packages/absl/app.py", line 254, in _run_main
    sys.exit(main(argv))
             ~~~~^^^^^^
  File "/Users/nicholasjunge/Workspaces/c++/benchmark/.venv/lib/python3.13/site-packages/google_benchmark/__init__.py", line 132, in _run_benchmarks
    return _benchmark.RunSpecifiedBenchmarks()
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/nicholasjunge/Workspaces/c++/benchmark/bindings/python/google_benchmark/example.py", line 91, in custom_counters
    num_foo, Counter.kIsRate | Counter.kInvert
             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/enum.py", line 1590, in __or__
    return self.__class__(value | other_value)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/enum.py", line 722, in __call__
    return cls.__new__(cls, value)
           ~~~~~~~~~~~^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/enum.py", line 1197, in __new__
    raise exc
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/enum.py", line 1174, in __new__
    result = cls._missing_(value)
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/enum.py", line 1469, in _missing_
    raise ValueError(
    ...<2 lines>...
                ))
ValueError: <flag 'Flags'> invalid value -2147483647
    given 0b1 0000000000000000000000000000001
  allowed 0b1 0000000000000000000000000001111