fix(windows/#3151): Support UNC paths when changing directories

[bryphe]

Issue: Trying to :cd into a UNC Path on Windows would fail

Defect: #3142 moved away from using strings to strongly-typed paths (Fp.t), to help ensure consistent normalization. However, the parsing logic for ingesting a string -> option(Fp.t) did not handle parsing UNC paths, so the system would treat it as a failed path.

Fix: Ideally, Fp could handle this out of the box: reasonml/reason-native#262

In the meantime, add an 'adapter' layer around Fp - when ingesting strings for Windows, first try to see if it is a UNC-style path. If it is, grab the server and share name and store it around an Fp path. Proxy all operations to and from the Fp path.

When the Fp is converted back to a string, make sure to use backslashes for that type of path.

Next steps:

• Considering porting fix to Fp(after some dependent PRs are in, like fix(fp): Remove Unix and Str dependencies reasonml/reason-native#261)

Fixes #3151