CLN: Consolidate numba facilities
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Precursor to https://github.com/pandas-dev/pandas/issues/31845 and other numba engine additions, creates `pandas/core/numba_.py` (open to move elsewhere) as a shared place for common numba operations like default parameters and jitting functions.
Maybe better placed in pandas/core/util? @jreback  Good point. I can move it to util.  seems good enough place for now. we can see how / if this grows and move around later. e.g. this is conceptually like pandas/core/computation stuff (though not nearly as big).