Add the ability to access the return value of a function through the VM Service when execution is paused on return

[DanTup]

In JS in VS Code, when you step over a return, there's an final stop location at the return which includes the return value in the variables list:

js.mp4

In Dart, we have a similar stop location (it jumps to before the return keyword):

dart.mp4

However the value being returned is not available through the VM Service. It would be useful for debugging to allow this value to be inspected without having to introduce an additional local variable.

(Originally filed at Dart-Code/Dart-Code#5104)

[rmacnak-google]

This a specific case of the more general issue that the evaluation stack isn't reported as part of the frame state along with the named local variables.