A frequent request from the last Flutter developer survey was for an easier method of testing light/dark mode changes. Currently, a user needs to manually change the theme settings or adjust phone settings to see the difference. Instead we should add a toggle from the CLI, and eventually devtools/Intellij/Vscode that allows developers to override the current setting.

Fixes #59495

Adds flutter.ext.brightnessOverride service protocol which either queries the current platform brightness, or overrides it to a new value. This accepts either Brightness.light or Brightness.dark as a value.

Adds a CLI toggle b which allows the setting to be toggled manually.

Requires an update to the MediaQuery, to conditionally use a debug override when not in release mode