Test: debug focus API

[connor4312]

Refs: #63943

• anyOS @lramos15
• anyOS @sandy081

Complexity: 3

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We've added a debug.onDidChangeActiveStackItem event that can be used to track the focused debug session.

1. Create a hello world extension.
2. Add a listener for debug.onDidChangeActiveStackItem and observe the value of debug.activeStackItem.
3. Start debugging your extension, and then in that window, start a couple other debug sessions (you can use the Debug: Open Link command as an example).
4. Validate the value and event behave as expected, and read through the docstrings and make sure they makes sense. The active stack item should change whenever a debug session pauses (and becomes active) or when the user selects another session in the Call Stack view. Ending the active session should focus another session, or undefined if there are no others.