Startup: "Forget" unused interpreters, or otherwise start them up less aggressively

[jmcphers]

Feedback from @nealrichardson:

I am working on an R package in Positron. There's no Python code in this directory anywhere, it's very clearly an R package. But when I open it in Positron, the Console tab is showing a Python interpreter running. I'm all for being Python first and all, but this seems excessive :)
Just to be sure, I just installed the latest Positron prerelease, and watched closely as the project opened. First it opened the Python repl, then it switched to the R console, and then after another little while, it switched back to Python

via https://positpbc.slack.com/archives/C05M2EZCPGR/p1716471902486219

This is emergent behavior resulting from two different Positron behaviors.

• Python is started if nothing else seems appropriate (this probably happened at one point in the past in Neal's project)
• We remember what was started and always start it again until you shut it down manually at some point

Both were intentional, but this emergent behavior isn't. We could help cut the chatter by:

• automatically un-affiliating runtimes that don't actually get used (when there is a runtime that does get used), and/or
• not creating an affiliation for a runtime that was only a guess

(Note that the switch-back-and-forth issue at startup is tracked separately, in #1143)