fix compile error

extensions/markdown-language-features/src/test/inMemoryDocument.ts

@@ -22,6 +22,7 @@ export class InMemoryDocument implements vscode.TextDocument {
 	isDirty: boolean = false;
 	isClosed: boolean = false;
 	eol: vscode.EndOfLine = vscode.EndOfLine.LF;
+	notebook: undefined;
 
 	get fileName(): string {
 		return this.uri.fsPath;
@@ -66,4 +67,4 @@ export class InMemoryDocument implements vscode.TextDocument {
 	save(): never {
 		throw new Error('Method not implemented.');
 	}
-}
\ No newline at end of file
+}