Feature request: Shadow DOM support

Describe the feature you'd like:

Web components are an increasingly popular way of creating reusable elements. Due to the existence of shadow DOM, user-focused testing is more difficult and requires tooling focused around the issue, as in the most permissive case, element queries/observer events (eg. via MutationObservers) stop at shadow boundaries and must explicitly be projected into shadow trees. Without support from tooling, aspects of a web app written using web components are virtually untestable without using methods contrary to the goals of this project.

Suggested implementation:

• Standard element queries can be modified to be projected into successive shadow trees
• Mutation observers to support eg. waits for DOM tree changes can be attached to all shadow roots of descendants
• Solution must consider related concepts (eg. slots) insofar as they relate to testing goals and principle of least surprise according to their documented behaviour/DOM standards
• No expectation of closed shadow roots to be supported in any way, but ideally would be explicitly documented

Describe alternatives you've considered:

• As first-class support is required, alternatives involve using/writing an alternative implementation which supports shadow trees explicitly (or potentially, writing a plugin system to allow others to support it)
• As shadow tree concepts are an aspect of WHATWG/W3C DOM standards, it makes sense that they're supported first-class by tooling that supports DOM operations

I'd be happy to add this feature, and support for basic functionality (query, waits) was straightforward to implement, but I wanted to get insight on constraints/concerns in advance as I don't currently use this library.

Start by tracing the library's element-query and wait/MutationObserver entry points, then review how shadow roots and slots should affect traversal. Done means defining and implementing an agreed approach for open shadow roots, waits, slots, and documentation of closed-root behavior, with coverage for the supported cases.