chore: Add subscription example (#2480)

* Add subscription example

* update example

* move out the sub source

examples/subscription/pages/index.js

@@ -0,0 +1,39 @@
+import React from "react"
+import useSWRSubscription from "swr/subscription"
+import EventEmitter from "events"
+
+const event = new EventEmitter()
+
+// Simulating an external data source.
+let stopped = false
+async function start () {
+  for (let i = 0; i < 100; i++) {
+    await new Promise(res => setTimeout(res, 1000))
+    if (stopped) return
+    if (i % 3 === 0 && i !== 0) {
+      event.emit("error", new Error("error: " + i));
+    } else {
+      event.emit("data", "state: " + i);
+    }
+  }
+}
+
+export default function page() {
+  const { data, error } = useSWRSubscription('my-sub', (key, { next }) => {
+    event.on("data", (value) => next(undefined, value));
+    event.on("error", (err) => next(err));
+    start();
+    return () => {
+      stopped = true;
+    };
+  })
+
+  return (
+    <div>
+      <h3>SWR Subscription</h3>
+      <h4>Received every second, error when data is times of 3</h4>
+      <div>{data}</div>
+      <div>{error ? error.message : ""}</div>
+    </div>
+  )
+}