Custom editors: extension development is hard due to our way of treating dirty

[bpasero]

Refs #91325

When testing this I had to configure my launch config with --extension-development-confirm-save to get confirm dialogs on close at least.

However, backup is never running in an extension host development window. We probably need to document this or revisit this approach.

[mjbvz]

Notes:

• Different behavior for tests vs running extension
• Make sure debug stop and cmd+q never prompts
• Investigate how backups work in extension development

[bpasero]

I pushed a change to:

• enable the save/revert dialog when closing a dirty file (unless running tests)
• enable hot-exit in-memory [1] even in extension development window which allows to still close the window without getting asked to save

[1]: this means that we execute the backup code but actually do not persist it anywhere on disk which I think is fine for the scenario of extension development. because there, the extension will be asked to persist the data for backup and can do so accordingly. upon restart - since the custom editor is likely still open - we will restore it and thus the extension will have a chance to test restoring from backup too.

I think this is a good compromise, let me know otherwise.

[Gama11]

Another reason why #25159 would be beneficial? ;)