Add k6 browser page.on method docs

src/data/markdown/docs/02 javascript api/07 k6-experimental/01 browser/10 Page/on.md

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+---
+title: 'on(event, handler)'
+excerpt: 'Browser module: page.on method'
+---
+
+Allows the user to define an event handler function that will be executed every time the specified event is emitted.
+
+| Parameter       | Type   | Default | Description                                                                                                                                                                                                                           |
+|-----------------|--------|---------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| event        | string  | `''`    |  Event to attach handler to. Currently, only `'console'` event is supported.                                                                                                               |
+| handler        | function  | `null`    |  A function to be called every time the specified event is emitted.                                                                                                               |
+
+
+<Blockquote mod="warning" title="">
+
+When using `page.on` method, the page has to be explicitly [closed](/javascript-api/k6-experimental/browser/page/close/) for the iteration to be able to finish.
+
+</Blockquote>
+
+
+### on('console')
+
+Emitted every time the console API methods are called from within the page JavaScript context.
+The arguments passed into the handler are defined by the [`ConsoleMessage`](/javascript-api/k6-experimental/browser/consolemessage) class.
+
+
+### Example
+
+<CodeGroup labels={[]}>
+
+```javascript
+import { browser } from 'k6/x/browser';
+import { check } from 'k6';
+
+export const options = {
+  scenarios: {
+    ui: {
+      executor: 'shared-iterations',
+      options: {
+        browser: {
+            type: 'chromium',
+        },
+      },
+    },
+  },
+  thresholds: {
+    checks: ["rate==1.0"]
+  }
+}
+
+export default async function() {
+  const page = browser.newPage();
+
+  try {
+    await page.goto('https://test.k6.io/');
+
+    page.on('console', msg => {
+        check(msg, {
+            'assertConsoleMessageType': msg => msg.type() == 'log',
+            'assertConsoleMessageText': msg => msg.text() == 'this is a console.log message 42',
+            'assertConsoleMessageArgs0': msg => msg.args()[0].jsonValue() == 'this is a console.log message',
+            'assertConsoleMessageArgs1': msg => msg.args()[1].jsonValue() == 42,
+        });
+    });
+
+    page.evaluate(() => console.log('this is a console.log message', 42));
+  } finally {
+    page.close(); // required so iteration can end
+  }
+}
+```
+
+</CodeGroup>