ENH: added regex argument to Series.str.split
- [X] closes #43563 
- [X] closes #32835
- [X] closes #25549
- [X] xref #37963 
- [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

I've preserved current behavior, in which regex = None. Currently, it handles the pattern as a regex if the length of pattern is not 1. I believe that in the future, this may be worth considering deprecating. 
