Improve startup performance & status

[jmcphers]

This change improves the startup experience in the Positron Console and also contains performance improvements that significantly decrease startup time (especially in dev builds)

Before the change, the console would say "Starting..." / "Starting up..." for several seconds before a console opened.

positron-before.mov

After it, in most Positron sessions no noticeable time will be spent in either state; instead, Positron will show you the interpreter it's working on starting.

positron-after.mov

Still more we could do to make this better, such as improving the styling and coordinating better with console startups, but want to minimize overlap with multi-console work.

Addresses #3566.

Release Notes

New Features

• Speed up console startup and show which interpreter will start (Show richer startup status in the Console #3566)

Bug Fixes

• N/A

QA Notes

You will see the new experience when Positron knows what runtime to start, but mostly the old one when Positron has to discover all interpreters (though even discovery will be a little faster now since it starts earlier). Specifically, you should see it:

• when a runtime is affiliated with the workspace
• when R or Python recommend a runtime for the workspace (currently NYI but will be implemented in those language packs in e.g. R: implement recommendedWorkspaceRuntime() #6208.
• when reloading a window

Test tags: @:console

[github-actions]

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

^{readme  valid tags}