Allow switching between legacy and new Inspector

[elliette]

Fixes #8269

Allow users to switch between the legacy inspector and the new inspector.

Previously the feature flag inspectorV2 determined whether to show the legacy InspectorScreen or the new InspectorScreen.

Now, the same InspectorScreen (inspector_shared/inspector_screen.dart) is always shown, and the user preference enableInspectorV2 determines whether it builds the legacy InspectorScreenBody or the new InspectorScreenBody.

Note: The InspectorSettingsDialog which had two implementations in the legacy inspector/ directory and the new inspector_v2/ directory now have been combined and also live in inspector_shared/.

[kenzieschmoll]

DBC: if we want to increase our chances of users using the V2 inspector to gather early feedback, wdyt about pulling the setting out into the top level row of actions as a Switch "Try the new Inspector!"?

[kenzieschmoll]

super-nit: can we take a screenshot without the debug label in the upper left? most users will experience this in the embedded view anyway so perhaps add the embedMode=single query param and then take a screenshot?

[elliette]

Yes! Will update the screenshots in the release notes as a last step once everything else in the PR looks good

[kenzieschmoll]

could this all be in the same spread?

if (controller != null && FeatureFlags.inspectorV2) ...[
  const SizedBox(width: defaultSpacing),
  ShowImplementationWidgetsButton(controller: controller!),
  const Spacer(),
  const SizedBox(width: defaultSpacing),
  SwitchSetting(...),
] else 
  const Spacer(),

[elliette]

Unfortunately it can't! The existence of controller means we are on new Inspector, but even for the legacy Inspector we need to show the SwitchSetting (so that users can enable the new Inspector)