Support Isolate Groups across DevTools

As of Dart 2.15, isolates can exist in an [isolate group](https://dart.dev/guides/language/concurrency#performance-and-isolate-groups). Isolates in a group share executable code and memory. We will need to take this into consideration for the Memory screen as well as other parts of DevTools where we currently assume that we are working with a single isolate (CPU profiler, isolate selector). We will likely also need to update the IsolateManager class to know about isolate groups.