worker: add connect and setConnectionsListener

[ShogunPanda]

This PR adds two new API to worker_threads that allow for cross-thread communication via MessagePorts.

A thread can invoke worker.connect to start a new connection to another thread. This method is blocking. Upon success, the return value is a MessagePort which can be used to exchange messages.

The core idea is that a thread willing to accept connections from other thread uses worker.setConnectionsListener to install a callback that it is invoked with a thread id, a port and (optional) data each time another thread attempts a connection.
The listener can return true to accept the connection. Any other return value will result in the connection being refused.
By default, if a thread has no listener associated, the connection will be refused.

Notable Change Text

A new set of experimental APIs has been added to worker_threads: connect and setConnectionsListener. These APIs aim to simplify 1-1 inter-thread communication in Node.js

Every thread (including the main one) can start a connection to any another thread (including the main one) using the connect API and providing the target threadId.
If the connection is successful, the call will return a MessagePort that can be used for the communication.

A thread can opt-in to receive incoming connections by calling the setConnectionsListener API. The listener will be invoked for each connection attempt and must return true to accept the connection. A thread without a connection listener will refused any connection by default.