test: add tests for lib/polyfill module

Add tests verifying that DisposableStack and AsyncDisposableStack
are properly polyfilled in globalThis and their basic functionality works.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lambdalisue

denops/fall/lib/polyfill_test.ts

@@ -0,0 +1,65 @@
+import { assertEquals, assertInstanceOf } from "jsr:@std/assert@^1.0.0";
+import { describe, it } from "jsr:@std/testing@^1.0.0/bdd";
+
+import "./polyfill.ts";
+
+describe("polyfill", () => {
+  it("should provide DisposableStack in globalThis", () => {
+    // deno-lint-ignore no-explicit-any
+    const stack = new (globalThis as any).DisposableStack();
+    assertInstanceOf(stack, Object);
+    assertEquals(typeof stack.dispose, "function");
+    assertEquals(typeof stack.use, "function");
+    assertEquals(typeof stack.adopt, "function");
+    assertEquals(typeof stack.defer, "function");
+    assertEquals(typeof stack.move, "function");
+
+    // Clean up
+    stack.dispose();
+  });
+
+  it("should provide AsyncDisposableStack in globalThis", () => {
+    // deno-lint-ignore no-explicit-any
+    const stack = new (globalThis as any).AsyncDisposableStack();
+    assertInstanceOf(stack, Object);
+    assertEquals(typeof stack.disposeAsync, "function");
+    assertEquals(typeof stack.use, "function");
+    assertEquals(typeof stack.adopt, "function");
+    assertEquals(typeof stack.defer, "function");
+    assertEquals(typeof stack.move, "function");
+
+    // Clean up
+    stack.disposeAsync();
+  });
+
+  it("should allow using DisposableStack functionality", () => {
+    // deno-lint-ignore no-explicit-any
+    const DisposableStack = (globalThis as any).DisposableStack;
+    const stack = new DisposableStack();
+    let disposed = false;
+
+    stack.defer(() => {
+      disposed = true;
+    });
+
+    assertEquals(disposed, false);
+    stack.dispose();
+    assertEquals(disposed, true);
+  });
+
+  it("should allow using AsyncDisposableStack functionality", async () => {
+    // deno-lint-ignore no-explicit-any
+    const AsyncDisposableStack = (globalThis as any).AsyncDisposableStack;
+    const stack = new AsyncDisposableStack();
+    let disposed = false;
+
+    stack.defer(async () => {
+      await Promise.resolve();
+      disposed = true;
+    });
+
+    assertEquals(disposed, false);
+    await stack.disposeAsync();
+    assertEquals(disposed, true);
+  });
+});