Don't require return value backup

Fixes #91703

All custom editors should implement backup or throw an exception if they cannot for some reason.

src/vs/workbench/api/common/extHostWebview.ts

@@ -727,7 +727,7 @@ export class ExtHostWebviews implements ExtHostWebviewsShape {
 		return document._saveAs(URI.revive(targetResource));
 	}
 
-	async $backup(resourceComponents: UriComponents, viewType: string, cancellation: CancellationToken): Promise<boolean> {
+	async $backup(resourceComponents: UriComponents, viewType: string, cancellation: CancellationToken): Promise<void> {
 		const document = this.getDocument(viewType, resourceComponents);
 		return document._backup(cancellation);
 	}