[VSCode] Add 'Explain terminal output' command

Main implementation issue for [VS Code] Terminal Assistance with Duo Chat (&17312)

Problem to solve

As a developer seeking quick solutions, I want to be able to ask Duo Chat questions about my terminal output and get immediate help without leaving my workflow.

Proposed solution

Add a new 'explain terminal output' command to the terminal context menu when the terminal has a selection.

Should integrate with the UI changes and status checks form the linked issues.