Allow addressing particular terminals and terminal commands in chat attachments

[Tyriar]

Currently the terminal contexts #terminalSelection and #terminalLastCommand work purely off the active (most recent) terminal. Since everything used to live in the extension, this was much more difficult to accomplish as there's no unique addressable API such as Terminal.resource: URI that we could have used, and referencing a particular command was basically impossible due to not having access to CommandDetectionCapability in core.

We're moving all this stuff to core (core #252650, other tools #259260), so this is something we could do.

[Tyriar]

Notes:

• URI query could be command=<guid>, that guid is created for every terminal command tracked in CommandDetectionCapability and de/serialized appropriately on reload