feat: add Shadow DOM support for XPath queries

[xjasonli]

Support DOCUMENT_FRAGMENT_NODE (ShadowRoot) as valid root for XPath expressions. Includes namespace resolution fallback to host element since ShadowRoot.lookupNamespaceURI returns null.

Fix: #681

[DrRataplan]

Thanks for the PR! Just looking through it. It's a tricky one, since the XQuery DOM has no notion of document fragments. I do agree fontoxpath can just treat them as document nodes, with multiple children. No harm in that.

A test or two would be really useful here. Slimdom has support for document fragments, which is I think the gist of this PR: supporting document fragments. A test for the sibling axis might be specifically interesting: creating a document fragment with two elements, and addressing the second one from the first with following-sibling::element(). Should work.

Can we rename the commits to supporting Document Fragments? I first thought we were implementing going from a shadow host into its shadowroot, exposing the shadowroot as a child of its host. Not sure if we want that.

KR,

Martin

[xjasonli]

Hi Martin

Thanks for the suggestions!

I just force-pushed the updates. I added some tests for document fragments, and the sibling axis one is working great. I also renamed the commit to "Support document fragments as root/context node" to keep things clear.

Let me know what you think!