ENH: add if_sheet_exists='overlay' to ExcelWriter
- [x] closes #42221, related to #40231
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry - not applicable

I mainly wanted this functionality myself, and am very open to input :)