feat(JitsiConference) add dispose method

It's a short-hand for leaving the conference and then disconnecting the
underlying connection.

In the general case (no breakout rooms) this is likely a preferred way
to handle leaving, since there is no good reason to keep the connection
open afterwards.

JitsiConference.js

     }
 };
 
+/**
+ * Disposes of conference resources. This operation is a short-hand for leaving
+ * the conference and disconnecting the connection.
+ */
+JitsiConference.prototype.dispose = async function() {
+    await this.leave();
+    await this.connection?.disconnect();
+};
+
 /**
  * Returns <tt>true</tt> if end conference support is enabled in the backend.
  *