Disallow navigating away from custom editors when retainContextWhenHidden is false and the backup has yet to run orfailed

Repro 1 (backup not yet run): 1. Create a custom editor extension that uses retainContextWhenHidden: false (the default) 2. Make some change that triggers a `CustomDocumentEditEvent` 3. Switch to another tab immediately (before backup runs) 4. Switch back Repro 2 (backup failed): 1. Create a custom editor extension that uses retainContextWhenHidden: false (the default) 2. Make some change in the custom editor which caused an error to be thrown in `CustomEditorProvider.backupCustomDocument` 4. Switch to another tab 5. Switch back The custom editor provider will have no option but to load the last known backup or from disk, which differs from the previous state. Resulting in data loss ☹️ For repro 1 https://github.com/microsoft/vscode/issues/113507 would be ideal, but tab switching should be blocked and a `backupCustomDocument` request issued immediately (allow maybe ~1 second for it to compete?). For repro 2, navigating away from the tab should be blocked with a dialog to save, don't save or cancel.