Obtain CWD/PWD

Hey!

I was looking through the source files and could, thus far, not find out if cppfs supported obtaining the current working directory. Unfortunately, there are a lot of information on how to use `getcwd()` on Windows - in fact, multiple versions... This is very confusing, so I hoped to find a simple method right here in cppfs, which I like a lot from what I saw so far! :)

So if a `getcwd()`-ish function is not implemented yet, I'd like to request it. :)

Kind regards, Ingwie.

The issue does not name files, tests, or an entry point. Start by tracing cppfs's public filesystem API and its platform-specific backends, then compare how each platform obtains the current working directory; done means a simple cross-platform API is available and covered by appropriate tests.

Written by the indexing model from the issue text.