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?

Start by examining the VM developer tools page and the existing VM Tools  Isolates  Open Ports feature. Read issues 844 and 5581 to understand prior context and whether the async-leak tool affects placement. Done means the open-ports feature is available in main DevTools rather than requiring VM developer mode.

Written by the indexing model from the issue text.