build: add jest, tests

tests/conditionalJoin.test.ts

@@ -0,0 +1,10 @@
+import { expect, test } from "@jest/globals";
+import { conditionalJoin } from "../src";
+
+test("conditionally joins strings", () => {
+  let isAdmin = true;
+
+  expect(
+    conditionalJoin(["Hello,", isAdmin ? "Administrator!" : "User!"], " "),
+  ).toBe("Hello, Administrator!");
+});