Pull "Open Ports" feature out into main DevTools

In the VM developer tools page, there is a feature to debug open ports (`VM Tools --> Isolates --> Open Ports`). This is useful for the general developer, not just the VM developer. For example, debugging why a test won't terminate. Here's a request for this feature that we've closed, although access to the feature requires the user to know that "VM developer mode" must be enabled: https://github.com/flutter/devtools/issues/844.

One question is where this feature would live. A related request is for a tool in DevTools to track leaking async operations (https://github.com/flutter/devtools/issues/5581) - maybe these two features are related and could live in a similar location?