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

先阅读当前的 DTDTools 屏幕和 Advanced Developer Mode 设置，然后查看阻塞性的 DevTools issue #7594 以及 package:dtd 扩展的要求。完成的标准是：该屏幕由 package:dtd 作为全局 DevTools 扩展提供，而不是继续隐藏在临时设置之后。

由索引模型根据 Issue 内容生成。