VfsPath: Add 'is_root()` method

[Property404]

This new method allows checking if a path is root without cloning or constructing anything.

Currently, I believe you have to do either:

• path.as_str().is_empty() which is less semantically obvious and exposes the implementation
• path == path.root() which is a bit more expensive because cloning an Arc requires atomic operations.

Previously, you could also do:

• path.parent().is_none() which had similar issues to path == path.root()