ENH: Add numba engine to rolling/expanding.std/var
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Failure due to codecov not being able to tell the numba decorated function is inherently tested. > is the arm failure real?

It's this flaky test

```
@pytest.mark.single
    def test_unique_bad_unicode(index_or_series):
        # regression test for #34550
        uval = "\ud83d"  # smiley emoj
``` thanks @mroeschke 