Cell attachment cleanup tool improvement in diff editor (#161132)

* Move attachment out of custom metadata, prep for attachment clean up in diff editor

* recover attachments from dirty notebook document

* Allow metadata to be restored when content changed/reverted in nb diff editor

src/vscode-dts/vscode.proposed.diffContentOptions.d.ts

@@ -0,0 +1,16 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+declare module 'vscode' {
+	// TODO@rebornix: add github issue link
+
+	export interface NotebookDocumentContentOptions {
+		/**
+		 * Controls if a cell metadata property should be reverted when the cell content
+		 * is reverted in notebook diff editor.
+		 */
+		cellContentMetadata?: { [key: string]: boolean | undefined };
+	}
+}