feat: remove dynamic require() with wsEngine

This change is necessary to get rid of:

> Critical dependency: the request of a dependency is an expression

when bundling the server with webpack.

BREAKING CHANGE: the syntax of the "wsEngine" option is updated

Before:

```js
const eioServer = require("engine.io")(httpServer, {
  wsEngine: "eiows"
});
```

After:

```js
const eioServer = require("engine.io")(httpServer, {
  wsEngine: require("eiows").Server
});
```

Related: https://github.com/socketio/engine.io/issues/609