ENH: Add support for writing variable labels to Stata files
- [x] closes #13536
- [x] closes #13535
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Add support for writing variable labels
Fix documentation for to_stata
Clean up function name to improve readability

closes #13536
closes #13535
