ENH: Allow literal (non-regex) replacement using .str.replace #16808
Adds `regex=True` default parameter to .str.replace. If `regex=False` is set, uses str.replace to replace occurrences of `pat` with `repl`. Raises `ValueError` if given a callable `repl` or a compiled re for `pat` in combination with `regex=False`.

- [ ] closes #16808
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
