Add cancellable async timeout utilities from VS Code core

[Copilot]

Brings cancellable async utilities from VS Code core (src/vs/base/common/async.ts) to improve async operation control and cancellation support.

Added APIs

• CancelablePromise<T> - Promise interface with .cancel() method
• CancellationError - Dedicated error class for cancellation scenarios
• createCancelablePromise<T>(callback) - Create promises cancellable via token or .cancel()
• timeout(millis) / timeout(millis, token) - Enhanced with cancellation support (backward compatible)
• raceCancellation<T>(promise, token, defaultValue?) - Race promise vs cancellation, resolves gracefully
• raceCancellationError<T>(promise, token) - Race promise vs cancellation, rejects with CancellationError

Usage

// Imperative cancellation
const t = timeout(1000);
t.cancel();

// Token-based cancellation  
await timeout(1000, cancellationToken);

// Race with graceful fallback
const result = await raceCancellation(operation, token, 'default');

// Race with error on cancel
await raceCancellationError(operation, token);

// Custom cancellable operations
const promise = createCancelablePromise(async (token) => {
    await longOperation(token);
    return result;
});
promise.cancel();

Existing timeout() calls remain unchanged. 23 unit tests added covering all new functionality.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Bring cancellable async timeout from core</issue_title>
<issue_description>From: #952 (comment)

Core has lots of useful functions like: https://github.com/microsoft/vscode/blob/2ca79c442ac3813c1531b8fea152299f64f012c7/src/vs/base/common/async.ts#L534

We should bring them too in this repo. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Bring cancellable async timeout from core #953

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