Node: make types.ts only export types rather than the entire class/code

[ibc]

Same as in versatica/mediasoup-client#273

NOTE: This was done in PR #1109 and later reverted here a9a34ca because it requires typescript >= 5 in the apps that import mediasoup-client.

This will be done again once we can assume that all users use typescript >= 5.

[ibc]

Closing. We do want to export real JS classes. Why not? We want to be able to do this:

import { WebRtcTransport } from 'mediasoup/lib/types';

transport instanceof WebRtcTransport;

This is not possible if we export WebRtcTransport as a type.