[VSCode] Add "explain terminal output" Code Lens

Problem to solve

As suggested in MR comment: !2516 (comment 2440868396)

Users will sometimes want to explain their terminal output in Duo Chat.

Adding a CodeLens action for this into the terminal may help discoverability of the feature and improve ease of use.

Proposed solution

Add a CodeLens for triggering the explain terminal output command. Ideally will only show when there is a selection in the terminal.