Outline transition

1. Finalize abstract-filepath package
2. find a place to put the core types https://github.com/hasufell/abstract-filepath/issues/9
3. Provide API variants for unix https://github.com/hasufell/abstract-filepath/issues/6 and Win32 https://github.com/hasufell/abstract-filepath/issues/7
4. Possibly provide API variants or alternatives for e.g. directory
5. Promote libraries to convert to new AbstractFilePath (users can still convert to e.g. FilePath if interfacing with other libraries)
6. Observe ecosystem effects, promote adoption in core libraries
7. Consider deprecating old FilePath completely