build: add jest, tests

tests/classNames.test.ts

@@ -0,0 +1,10 @@
+import { expect, test } from "@jest/globals";
+import { classNames } from "../src";
+
+test("joins html class strings", () => {
+  let isVisible = false;
+
+  expect(classNames("bg-blue-300", isVisible ? "block" : "hidden")).toBe(
+    "bg-blue-300 hidden",
+  );
+});