Use .ts files for vite config for type safety

vitest.config.ts

@@ -0,0 +1,32 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
+*/
+
+import { defineConfig, mergeConfig } from "vitest/config";
+
+import viteConfig from "./vite.config";
+
+export default defineConfig((configEnv) =>
+  mergeConfig(
+    viteConfig(configEnv),
+    defineConfig({
+      test: {
+        environment: "jsdom",
+        css: {
+          modules: {
+            classNameStrategy: "non-scoped",
+          },
+        },
+        setupFiles: ["src/vitest.setup.ts"],
+        coverage: {
+          reporter: ["html", "json"],
+          include: ["src/"],
+          exclude: ["src/**/*.{d,test}.{ts,tsx}", "src/utils/test.ts"],
+        },
+      },
+    }),
+  ),
+);