Suggestion: Show global variables in variable window

Currently the vm service protocol exposes globals per library in https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#library

We have an occasional ask from customers for exposing global variables for inspection in dart DevTools. Would it be possible to add them to the variable window (and separate globals and locals into different groups)?

VM service implementations currently support evaluation of globals (that are available in the current scope) in the evaluation box, so it seems feasible to show globals upfront.

Note: might need additional support from implementations of VmServiceInterface. On web platform, for example, globals are lazily in created in JavaScript and will require calling a getter. In this case we could show a `...` next to a global name and call the getter on click.