Add markdown cell to Notebook (#3014)

* add markdown cell

* add markdown preview for Notebook

* formatting

* address comment

extensions/markdown-language-features/src/commands/showNotebookPreview.ts

@@ -0,0 +1,20 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+import * as vscode from 'vscode';
+
+import { Command } from '../commandManager';
+import { MarkdownEngine } from '../markdownEngine';
+
+export class ShowNotebookPreview implements Command {
+	public readonly id = 'notebook.showPreview';
+
+	public constructor(
+		private readonly engine: MarkdownEngine
+	) { }
+
+	public async execute(document: vscode.Uri, textContent: string): Promise<string> {
+		return this.engine.renderText(document, textContent);
+	}
+}