Make DTDTools screen a DevTools extension

This screen was temporarily added behind the "Advanced Developer Mode" setting to support inspecting DTD instances. Ideally, this should be shipped as a DevTools extension provided by `package:dtd` as a global devtools extension, which is blocked on https://github.com/flutter/devtools/issues/7594.

Putting the screen behind the "Advanced Developer Mode" setting is okay for now, but is not the best permanent solution. CC @bkonyi