Support nesting threads in the debugger

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> This is a companion to microsoft/debug-adapter-protocol#339. In conjunction with that proposal, when debugging a process via a debug adapter that exposes the parent-child relationship between threads, I want VSCode to show threads in a tree. It probably should be opt-in or opt-out via a setting or something. ### Motivation Go uses [green threads](https://en.wikipedia.org/wiki/Green_thread), called goroutines. A Go release in the near future will provide information from the debugger than can be used to determine a parent-child relationship between goroutines. Complex servers may have hundreds or thousands of goroutines. Debugging such a server can be challenging, especially when attempting to hunt down what caused a goroutine to be launched. I want Go's debug adapter, DAP, and VSCode to be updated to provide me the option of nesting goroutines to both make the call stack panel more manageable and make it easier to track down the ultimate cause of an event.