[devtools-shell] layout options for testing (#33516)

## Summary

This PR adds a 'Layout' selector to the devtools shell main example, as
well as a resizable split pane, allowing more realistic testing of how
the devtools behaves when used in a vertical or horizontal layout and at
different sizes (e.g. when resizing the Chrome Dev Tools pane).

## How did you test this change?



https://github.com/user-attachments/assets/81179413-7b46-47a9-bc52-4f7ec414e8be