Specify behavior of `*Path::RelativeTo` for cases when it's impossible to build a relative path

On Unix file system, it's always possible to build a relative path between two paths. On Windows, however, it is not universally possible: say, between different disks or between a DOS device and a disk. The latter aspect often surprises people when they use `java.nio.file.Path::relativize` (that throws an exception), so we better do something else: either show the nature of the method in its type, or invent an alternate behavior.

Also, it is not possible between network shares.

We should decide and document the behavior of this method for these cases.

I am thinking about leaving a strict method that would return `null` for cases when it was unable to build a path, and a separate lax method like `RelativeToIfPossible` that would return `this` for such cases.