backupCustomDocument
Back up a dirty custom document.
Backups are used for hot exit and to prevent data loss. Your backup method should persist the resource in its current state, i.e. with the edits applied. Most commonly this means saving the resource to disk in the ExtensionContext.storagePath. When the editor reloads and your custom editor is opened for a resource, your extension should first check to see if any backups exist for the resource. If there is a backup, your extension should load the file contents from there instead of from the resource in the workspace.
backup is triggered approximately one second after the user stops editing the document. If the user rapidly edits the document, backup will not be invoked until the editing stops.
backup is not invoked when auto save is enabled (since auto save already persists the resource).
Parameters
Document to backup.
Information that can be used to backup the document.
Token that signals the current backup since a new backup is coming in. It is up to your extension to decided how to respond to cancellation. If for example your extension is backing up a large file in an operation that takes time to complete, your extension may decide to finish the ongoing backup rather than cancelling it to ensure that the editor has some valid backup.