chore: migrate to rollup

This change allows us to:

- reduce the size of the bundle
- provide an ESM bundle (for usage in <script type="module">)

BREAKING CHANGE: due to how default export works with ES modules, we
have removed the default export of the library, which means the
following code:

```js
require("engine.io-client")(...);
```

will not work anymore. The named export must be used instead:

```js
const { Socket } = require("engine.io-client);
// or import { Socket } from "engine.io-client";

const socket = new Socket(...);
```

Note: the UMD build still exposes a function though:

```html
<script src="/path/to/engine.io.js"></script>
<script>
  const socket = eio(...);
</script>
```

Note: webpack is still used with zuul because of the custom builder
(zuul-builder-webpack)