Support editing textScaleFactor, brightness, etc. in devtools

The ability to switch between dark/light mode, adjust text scale, switch target platform (iOS, Android, etc.) would allow devs to quickly verify that their widget functions as expected in a variety of environments.

To illustrate the benefit this would provide, the current flow for switching between dark/light mode on an iOS simulator is:
- Open the Settings app
- Open the Developer page
- Toggle "Dark appearance"

Even worse, changing text scale:
- Open the Settings app
- Open the Accessibility page
- Open the Display & Text Size page
- Open the Larger Text page
- Drag the slider up or down to adjust text scale

Having a way to do this without so many clicks and without having to leave the app would be phenomenal.

https://pub.dev/packages/device_preview demonstrates a version of the functionality that would be great to have as part of DevTools.

issue 中没有指定源文件或测试。首先检查用于 runtime 或设备配置的 DevTools 入口点，并与链接的 device_preview package 进行比较；定义应如何暴露深色/浅色模式、文本缩放、亮度和目标平台。完成的标准是，开发者可以在不离开应用的情况下通过 DevTools 更改这些环境设置。

由索引模型根据 Issue 内容生成。