Support for UI Toolkit

**Describe the feature/enhancement**

Extend AltTester to support the new UI Toolkit

**What is the reason you need this feature?**

I developed a lot of UI heavy cross-platform apps. The new Unity UI system finally brings some changes that simplify development and I already use it in a project. Now we also want to use AltTester in this project. I modified AltTester to return Visual Elements instead of GameObjects and get their screen position. However, I haven't been working with AltTester long enough to fully integrate it e.g. as AltVisualElement. Are there any plans for integration?

/label ~enhancement