NodeTraversor class should be open instead of internal

[dioKaratzas]

Describe the issue

The NodeTraversor class is currently internal, which makes it unusable outside the module. Additionally, the traverse(_:) method is marked as open, which has no effect unless the class itself is open.

Steps to reproduce

1. Try to subclass NodeTraversor in a project that imports SwiftSoup.
2. Notice that the class is inaccessible.

Expected behavior

The NodeTraversor class should be open so that it can be used and subclassed outside the module.

Proposed solution

Change:

internal class NodeTraversor

With:

open class NodeTraversor

This will align with the existing open func traverse(_:) declaration and allow proper subclassing.