Add vscode.env.webviewResourceRoot api

Fixes #72155

Adds a constant to the api that tracks the root path for resources inside of webviews. This is required because we will not be able to use `vscode-resource:` uris on the web. Our current approach is to rewrite the html we are given but there are almost certainly going to be cases where we don't get this quite right.

Adopts the new api for the markdown preview

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

@@ -0,0 +1,16 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as vscode from 'vscode';
+
+const rootUri = vscode.Uri.parse(vscode.env.webviewResourceRoot);
+
+export function toResoruceUri(uri: vscode.Uri): vscode.Uri {
+	return rootUri.with({
+		path: rootUri.path + uri.path,
+		query: uri.query,
+		fragment: uri.fragment,
+	});
+}