Windows support

Periscope doesn't currently work on Windows. The tests also don't work -- the tests using [testfs](https://github.com/anishathalye/periscope/tree/master/testfs) (which relies on [afero](https://github.com/spf13/afero/)'s MemFs) seem to hang / OOM, and the tests using the real filesystem and fancy features like symlinks don't work either.

To support Windows, we need to, among other things:

- Adjust how we handle detecting the root directory (see https://github.com/anishathalye/periscope/issues/10#issuecomment-1367892986)
- Figure out why the MemFs-based tests aren't working, and fix them
- Figure out why tests using the real filesystem aren't working, and either split them into separate files that are only used on macOS/Linux, or figure out how to adapt them to be cross-platform
- Understand any potential subtleties of Periscope on Windows/NTFS, fix the code to handle these cases, and add Windows-specific tests

(I don't use Windows, so this is a low-priority issue for me. There's a good chance this will only be supported if someone can help out with this.)