Additional information used to implement {@linkcode CustomEditableDocument.backup}.
Fields
finalread onlydestination:Uri
Suggested file location to write the new backup.
Note that your extension is free to ignore this and use its own strategy for backup.
If the editor is for a resource from the current workspace, destination will point to a file inside
ExtensionContext.storagePath. The parent folder of destination may not exist, so make sure to created it
before writing the backup to this location.