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.

No source files or tests are named in the issue. Start by reviewing the DevTools entry points for runtime or device configuration and compare the linked device_preview package; define how dark/light mode, text scale, brightness, and target platform should be exposed. Done means developers can change those environment settings from DevTools without leaving the app.

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