Add extended markdown notebook renderer sample

This demonstrates how one notebook renderer can extend another one

notebook-extend-markdown-renderer-sample/src/emoji.ts

@@ -0,0 +1,23 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+import type * as MarkdownIt from 'markdown-it';
+import type { RendererContext } from 'vscode-notebook-renderer';
+
+interface MarkdownItRenderer {
+    extendMarkdownIt(fn: (md: MarkdownIt) => void): void;
+}
+
+export async function activate(ctx: RendererContext<void>) {
+    const markdownItRenderer = await ctx.getRenderer('markdownItRenderer') as MarkdownItRenderer | undefined;
+    if (!markdownItRenderer) {
+        throw new Error('Could not load markdownItRenderer');
+    }
+
+    const emoji = require('markdown-it-emoji');
+    markdownItRenderer.extendMarkdownIt((md: MarkdownIt) => {
+        return md.use(emoji, {});
+    })
+}
+