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)

README.md

@@ -41,7 +41,8 @@ Engine.IO is a commonjs module, which means you can include it by using
 1. write your app code
 
     ```js
-    const socket = require('engine.io-client')('ws://localhost');
+    const { Socket } = require('engine.io-client');
+    const socket = new Socket('ws://localhost');
     socket.on('open', () => {
       socket.on('message', (data) => {});
       socket.on('close', () => {});
@@ -65,7 +66,7 @@ Engine.IO is a commonjs module, which means you can include it by using
 ```html
 <script src="/path/to/engine.io.js"></script>
 <script>
-  const socket = new eio.Socket('ws://localhost/');
+  const socket = eio('ws://localhost/');
   socket.binaryType = 'blob';
   socket.on('open', () => {
     socket.send(new Int8Array(5));
@@ -80,7 +81,8 @@ Engine.IO is a commonjs module, which means you can include it by using
 Add `engine.io-client` to your `package.json` and then:
 
 ```js
-const socket = require('engine.io-client')('ws://localhost');
+const { Socket } = require('engine.io-client');
+const socket = new Socket('ws://localhost');
 socket.on('open', () => {
   socket.on('message', (data) => {});
   socket.on('close', () => {});
@@ -95,7 +97,8 @@ const opts = {
   ca: fs.readFileSync('test/fixtures/ca.crt')
 };
 
-const socket = require('engine.io-client')('ws://localhost', opts);
+const { Socket } = require('engine.io-client');
+const socket = new Socket('ws://localhost', opts);
 socket.on('open', () => {
   socket.on('message', (data) => {});
   socket.on('close', () => {});
@@ -111,7 +114,8 @@ const opts = {
   }
 };
 
-const socket = require('engine.io-client')('ws://localhost', opts);
+const { Socket } = require('engine.io-client');
+const socket = new Socket('ws://localhost', opts);
 socket.on('open', () => {
   socket.on('message', (data) => {});
   socket.on('close', () => {});
@@ -124,13 +128,13 @@ Please note that in this case the headers won't be sent in the WebSocket upgrade
 
 ```js
 // WILL NOT WORK in the browser
-require('engine.io-client')('http://localhost', {
+const socket = new Socket('http://localhost', {
   extraHeaders: {
     'X-Custom-Header-For-My-Project': 'will not be sent'
   }
 });
 // WILL NOT WORK
-require('engine.io-client')('http://localhost', {
+const socket = new Socket('http://localhost', {
   transports: ['websocket'], // polling is disabled
   transportOptions: {
     polling: {
@@ -141,7 +145,7 @@ require('engine.io-client')('http://localhost', {
   }
 });
 // WILL WORK
-require('engine.io-client')('http://localhost', {
+const socket = new Socket('http://localhost', {
   transports: ['polling', 'websocket'],
   transportOptions: {
     polling: {

lib/browser-entrypoint.ts

@@ -0,0 +1,3 @@
+import { Socket } from "./socket.js";
+
+export default (uri, opts) => new Socket(uri, opts);

lib/index.ts

@@ -1,11 +1,5 @@
 import { Socket } from "./socket.js";
 
-export default (uri, opts) => new Socket(uri, opts);
-
-/**
- * Expose deps for legacy compatibility
- * and standalone browser access.
- */
 export { Socket };
 export { SocketOptions } from "./socket.js";
 export const protocol = Socket.protocol;