Fix @xterm/headless compatibility with Bun and Deno runtimes

Replace `window` with `globalThis` in IdleTaskQueue export.
In Bun and Deno, `isNode` is false but `window` is undefined, causing
a ReferenceError. Using `globalThis` is safe in all JS runtimes and
makes the `isNode` guard unnecessary.

This is the only `in window` expression in code bundled into
@xterm/headless.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/common/TaskQueue.ts

@@ -3,7 +3,6 @@
  * @license MIT
  */
 
-import { isNode } from 'common/Platform';
 import type { ILogService } from 'common/services/Services';
 
 interface ITaskQueue {
@@ -148,7 +147,7 @@ class IdleTaskQueueInternal extends TaskQueue {
  * This reverts to a {@link PriorityTaskQueue} if the environment does not support idle callbacks.
  */
 // eslint-disable-next-line @typescript-eslint/naming-convention
-export const IdleTaskQueue = (!isNode && 'requestIdleCallback' in window) ? IdleTaskQueueInternal : PriorityTaskQueue;
+export const IdleTaskQueue = ('requestIdleCallback' in globalThis) ? IdleTaskQueueInternal : PriorityTaskQueue;
 
 /**
  * An object that tracks a single debounced task that will run on the next idle frame. When called