docs: add a note about CodeSpaces UI mode (#23580)

Author: aslushnikov

docs/src/test-ui-mode-js.md

@@ -81,3 +81,13 @@ Click on the pick locator button and hover over the DOM snapshot to see the loca
 Next to the name of each test in the sidebar you will find an eye icon. Clicking on the icon will activate watch mode which will re-run the test when you make changes to it. You can watch a number of tests at the same time be clicking the eye icon next to each one or all tests by clicking the eye icon at the top of the sidebar. If you are using VS Code then you can easily open your test by clicking on the file icon next to the eye icon. This will open your test in VS Code right at the line of code that you clicked on.
 
 ![watch mode in ui mode](https://user-images.githubusercontent.com/13063165/234304918-dd0fb6d5-bfb1-4182-8c55-33cd3da5f83e.png)
+
+### Github CodeSpaces support
+
+To run UI mode inside Github CodeSpaces, run the following:
+
+  ```bash
+  npx playwright test --ui-port=0
+  ```
+
+After that, the UI mode will be automatically available to you in a separate browser tab, thanks to the [CodeSpaces automatic port forwarding](https://docs.github.com/en/codespaces/developing-in-codespaces/forwarding-ports-in-your-codespace#about-forwarded-ports).