Add JS toolchain: TypeScript, ESLint, Prettier, Vitest

package.json declares the project as ESM with NodeNext module resolution,
exposing dist/index.js as the library entry and dist/bin/quack.js as the
CLI binary. Dev dependencies are pinned to exact versions (yarn.lock holds
the integrity hashes per repo policy on hash-pinned external references).
Adds a placeholder src/index.ts and a single smoke test so make check is
not a no-op.

eslint.config.mjs

@@ -0,0 +1,10 @@
+import js from "@eslint/js";
+import tseslint from "typescript-eslint";
+
+export default tseslint.config(
+    {
+        ignores: ["dist/", "node_modules/", "coverage/", ".vitest-cache/"],
+    },
+    js.configs.recommended,
+    ...tseslint.configs.recommended,
+);