fix: restore the namespace export

This restores the previous behavior, where the "io" object available in
the browser could be used as a function (`io()`) or as a namespace
(`io.connect()`).

The breaking change was introduced in [1].

Related: socketio/socket.io#4121

[1]: 16b6569

lib/index.ts

@@ -71,6 +71,15 @@ function lookup(
   return io.socket(parsed.path, opts);
 }
 
+// so that "lookup" can be used both as a function (e.g. `io(...)`) and as a
+// namespace (e.g. `io.connect(...)`), for backward compatibility
+Object.assign(lookup, {
+  Manager,
+  Socket,
+  io: lookup,
+  connect: lookup,
+});
+
 /**
  * Protocol version.
  *