Fslib API

[arcanis]

What's the problem this PR addresses?

• The toPortablePath / fromPortablePath were exposed from both NodeFS and path.ts.
• There was no way to implement a partial FakeFS without implementing each and every method.
• Checking whether a path is contained in another path is common but annoying to reimplement.

How did you fix it?

• The toPortablePath and fromPortablePath are moved into npath. Callsites are updated.
• A new NoFS implementation will throw at runtime for each unimplemented method.
• Both npath and ppath now expose an utility called contains which returns null if the second parameter isn't contained in the first one, or the subpath otherwise.