Update ts configs, add diff-based linter.

.github/workflows/test-src.yml

@@ -21,4 +21,4 @@ jobs:
         run: npm run test
 
       - name: lint
-        run: npm run lint
+        run: npm run lint -- --base ${{ github.event.pull_request.base.ref }}

.prettierrc.js

@@ -0,0 +1,13 @@
+// Copyright (c) 2024 The Brave Authors. All rights reserved.
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// This file is placed at the top level instead of src/ to support formatting
+// for all files, not only in src/.
+
+/** @type {import("prettier").Config} */
+module.exports = {
+  plugins: ['prettier-plugin-organize-imports'],
+  singleQuote: true,
+};