feat: Add Symbol.dispose and Symbol.asyncDispose support to DispatcherBase

[PandaWorker]

This PR implements the new ECMAScript Explicit Resource Management proposal by adding disposal functionality to the DispatcherBase class. The changes include:

1. Added [Symbol.asyncDispose]() method that asynchronously closes the dispatcher
2. Added [Symbol.dispose]() method that synchronously closes the dispatcher
3. Both methods reuse the existing close() functionality
4. The synchronous version uses the noop function as a callback

These changes enable modern resource management patterns:

// Synchronous disposal
using dispatcher = new Agent();

// Async disposal
await using asyncDispatcher = new Client();

The implementation follows the TC39 proposal currently in Stage 3 (https://github.com/tc39/proposal-explicit-resource-management)

[mcollina]

I think this is a good start. You should make sure tha the result of .compose() can be disposed.

Also, docs are missing.

[PandaWorker]

I'm also confused about what 's after .the close() agent no longer accepts requests, I think we need to completely not accept requests only at .destroy(), or add a method .closeIdleConnections(), which will simply wait for requests to complete and close all connections.

[mcollina]

I'm also confused about what 's after .the close() agent no longer accepts requests

I agree, I think it should error at this point but not drop any current requests.