Don't start runtimes that were never used

[jmcphers]

This change cause Positron to "forget" runtimes that are affiliated with the workspace but that aren't actually used, with the goal of preventing unwanted sessions from starting.

Some details:

• A runtime is considered to be unused if it was auto-started as an affiliated runtime but then never actually used in the last session.
• The runtime affiliation is not wholly removed; we just stop auto-starting it when Positron starts. So e.g. it is still consulted when auto-starting a console in order to run code.
• This only applies to projects with multiple runtime affiliations. If you only have one runtime affiliated, it will auto-start even if you didn't use it in your last session.

Addresses #3432.

Release Notes

New Features

• Do not auto-start interpreters that aren't being used in the workspace (Startup: "Forget" unused interpreters, or otherwise start them up less aggressively #3432)

Bug Fixes

• N/A

QA Notes

For the purposes of this feature, a runtime is considered to have been "used" if you switch to it in the Console.

[github-actions]

E2E Tests 🚀  ^?
This PR will run tests tagged with: @critical

[timtmok]

LGTM