Stream Muxer should be allowed to return a promise

[MarcoPolo]

diff --git a/packages/interface-stream-muxer/src/index.ts b/packages/interface-stream-muxer/src/index.ts
index 6ad19dd..a0efad2 100644
--- a/packages/interface-stream-muxer/src/index.ts
+++ b/packages/interface-stream-muxer/src/index.ts
@@ -18,7 +18,7 @@ export interface StreamMuxer extends Duplex<Uint8Array> {
    * Initiate a new stream with the given name. If no name is
    * provided, the id of the stream will be used.
    */
-  newStream: (name?: string) => Stream
+  newStream: (name?: string) => Stream | Promise<Stream>
 
   /**
    * Close or abort all tracked streams and stop the muxer

Sometimes a stream muxer needs to do some async work before it can return a new stream (as is the case with webtransport). The current implementation already calls newStream from an async function, so it wouldn't be a big change. In fact, the connection interface already returns a Promise<Stream> when a user calls newStream(...), so this is inline with that async context.

[mpetrunic]

closed via #290