Provide XPath integration for XML documents

[FlorianRappl]

Description

Add an optional XPath integration layer for XML documents so users can execute XPath queries directly against AngleSharp XML DOM nodes.

Possible shape:

• extension methods on document and node
• select single node
• select node set
• namespace-aware evaluation context

This could live in the XML package or a companion package to keep dependencies optional.

Background

User scenarios include:

• migrating existing XPath-centric codebases to AngleSharp
• querying XML schemas, feeds, and domain documents where CSS selectors are insufficient
• reducing glue code between AngleSharp DOM and external XPath engines

This fills a common usability gap in XML-first workflows.

Specification

Relevant specifications:

• W3C XPath 1.0 (baseline interoperability target)
• Optional future alignment with XPath 3.1 depending on scope and dependency strategy
• XML Namespaces handling as required by XPath evaluation