Adjust os.Path.apply() to take relative paths and expand home

[Flowdalic]

OS-Lib was lacking a convenience methods that lets user construct absolute paths from a wide range of inputs values. In particular, there is no function that takes either an absolute path, relative path or a tilde prefixed path and constructs an absolute os.Path from it.

The new behavior of os.Path.apply() accommodates for this.

Although I am not sure if I understand the proposal made in https://github.com/com-lihaoyi/os-lib/pull/101#issuecomment-1112731311, this hopefully satisfies the proposal.