Fix escaping of raw values that contain & in md preview

Fixes microsoft#236660

Author: mjbvz

extensions/markdown-language-features/src/util/dom.ts

@@ -5,7 +5,10 @@
 import * as vscode from 'vscode';
 
 export function escapeAttribute(value: string | vscode.Uri): string {
-	return value.toString().replace(/"/g, '"');
+	return value.toString()
+		.replace(/&/g, '&')
+		.replace(/"/g, '"')
+		.replace(/'/g, ''');
 }
 
 export function getNonce() {