add vitest config

Author: uzmoi

vitest.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+	test: {
+		coverage: {
+			include: ['src'],
+		},
+		include: ['test/**'],
+		exclude: ['test/testutils.ts'],
+	},
+});