fix(@libp2p/webrtc): close data-channel on muxer stream end (#1976)

`RTCDataChannel`s are not properly closed after the stream ends. They stay in an open state and eventually the browser will not be able to create more `RTCDataChannel`s with the following error:

```
Uncaught (in promise) DOMException: Failed to execute 'createDataChannel' on 'RTCPeerConnection': RTCDataChannel creation failed
```

Fixes #1895

Author: Marcel-G

packages/transport-webrtc/src/muxer.ts

@@ -158,6 +158,7 @@ export class DataChannelMuxer implements StreamMuxer {
       direction: 'outbound',
       dataChannelOptions: this.dataChannelOptions,
       onEnd: () => {
+        channel.close() // Stream initiator is responsible for closing the channel
         this.streams = this.streams.filter(s => s.id !== stream.id)
         this.metrics?.increment({ stream_end: true })
         this.init?.onStreamEnd?.(stream)