Create a landing screen for profile mode and Dart CLI apps

[kenzieschmoll]

When opening DevTools for a Dart CLI app or a profile mode Flutter app, the Inspector page is not the first page to load. In these cases, the Performance page is the first page to load, and then the user is forced to pay the cost of loading the timeline, even if they do not want to open the performance page.

It may make sense to have a very simple landing page ( = fast IPL) in these modes that lets the user navigate to the specific tool they want. We could optionally add some information in VM developer mode that would be useful (for example, like some of the high level information on the Observatory landing page).