Add Warning Message When Renaming Active .ipynb Files

[c0m]

Currently, when a .ipynb file is renamed, VSCode starts a new kernel session without warning. This is abrupt and could disrupt user workflow, especially when working with large notebooks that take a significant amount of time to initialize.

This could be mitigated by having a warning message pop up when the user tries to rename an active .ipynb file.

This would reduce surprises if a user is changing file names while waiting for something in their notebook to complete.

Another potential solution could be to maintain the kernel session after the name change.

[amunger]

This also applies to saving an untitled file

[rebornix]

Let's merge this into microsoft/vscode-jupyter#11010