Add basic mermaid rendering support in core

For #257761

Ports over extension sample + a few improvements to core

extensions/mermaid-chat-features/extension-browser.webpack.config.js

@@ -0,0 +1,13 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+// @ts-check
+import { browser as withBrowserDefaults } from '../shared.webpack.config.mjs';
+
+export default withBrowserDefaults({
+	context: import.meta.dirname,
+	entry: {
+		extension: './src/extension.ts'
+	}
+});