fix: remove implicit require of uws

So that bundlers like webpack do not try to include it in the build.

As a side-effect, any implementation which matches the API of the ws
module can now be used.

Before that change, you had to explicitly exclude uws:

```
// webpack.config.js
module.exports = {
  // ...
  externals: {
     uws: 'uws'
  }
};
```

Related: #575

lib/server.js

@@ -69,17 +69,9 @@ class Server extends EventEmitter {
 
     if (this.ws) this.ws.close();
 
-    let wsModule;
-    switch (this.opts.wsEngine) {
-      case "uws":
-        wsModule = require("uws");
-        break;
-      case "ws":
-        wsModule = require("ws");
-        break;
-      default:
-        throw new Error("unknown wsEngine");
-    }
+    // add explicit require for bundlers like webpack
+    const wsModule =
+      this.opts.wsEngine === "ws" ? require("ws") : require(this.opts.wsEngine);
     this.ws = new wsModule.Server({
       noServer: true,
       clientTracking: false,