Implement persistent native interactive window sessions

[joyceerhl]

We recently implemented being able to restore the contents of the interactive window from backup between VS Code reloads. However, there are a number of issues that need to be resolved in order for this experience to work smoothly.

The current behavior:

• Reload
• Interactive window is still open with all the cells from the previous session
• Mapping between Python file and interactive window is lost
• Kernel is initially dead (killed on reload). After waiting a bit, the previously connected kernel gets restarted, and a new connection sys info cell is appended to the interactive window (weird behavior for users--user expectation here is that it's the same kernel connection)

Discussion topic: What is the meaning of hot exit, and why does it exist?
- Is it just about persisting the history in the interactive window, or is it also about having a persistent kernel connection?
- Currently the implementation supports the former, i.e. we keep the cells around but the kernel session is lost
- One perspective: if I reload VS Code, I don't want to lose my interactive window session--that means the mapping between the Python file and interactive window should still be there, and the kernel connection should still be live and I should not lose my kernel state #6420

Work items

• Persist a mapping between the active #%% Python file and the interactive window, and restore this mapping on reload Interactive window on reload will not associate with the same python file #6421
• Add support for explicit raw kernel session management (don't auto kill on reload) Support persistent raw kernel sessions #6420
• Ensure controller connection isn't lost Python controllers aren't registered fast enough #6419

[amunger]

Created a more up to date discussion thread for this