Migrate to use OSPath instead of FilePath

Hello! Let me start by saying thanks for this great library. I learned a lot by using it so far as a Haskell beginner. :smiley:

Anyways, I noticed that `filepath` package recommends to use [OSPath](https://hackage.haskell.org/package/filepath-1.5.2.0/docs/System-OsPath.html) instead of FilePath.

Since I believe that `turtle` is largely being used as a shell scripting alternative (including in cross-platform manner) and which means, may involve a lot of file and directory manipulations, I reckon it's probably better in long term to adopt OSPath?

I was inspired to open this issue after reading this [post](https://discourse.haskell.org/t/the-ultimate-guide-to-haskell-strings/9538) on Discourse.

Thanks!

The issue names no files, tests, or entry points; start by auditing turtle's file and directory APIs for FilePath usage and reading the linked filepath OSPath documentation. Done means defining and completing the migration to OSPath across the relevant APIs while preserving cross-platform shell behavior and updating coverage as needed.

Written by the indexing model from the issue text.