Add API for subscribing to code execution events

[jmcphers]

Positron should have an API that allows extensions to subscribe an event that fires each time the user runs code. The event should include

• the language in which code was executed
• the code that was executed
• any other relevant metadata

This is probably useful for many purposes, but the primary motivation is to make it possible for the Posit Workbench extension to build a feature that connects Positron to Posit Workbench's console auditing feature.

[jthomasmock]

Would this also be appropriate for Workbench to know when Positron is performing computation or not? Currently, Workbench doesn't have timeouts for VS Code, but if Workbench knows that Positron isn't actively computing something, we could have a pretty safe indication to allow timeout of a session.

[jmcphers]

No, I think that should be built out as a separate API, one that is query-based ("are you currently computing?") rather than event based.

[jthomasmock]

No, I think that should be built out as a separate API, one that is query-based ("are you currently computing?") rather than event based.

Makes sense. Thank you, I'll open a separate issue.