[Feature Request] Add a button for killing the preview iframe

[carlsmith]

When running demos in the Studio, there is no easy way to kill them while you do more work on the code. You can minimize the bottom pane, but it doesn't stop the demo running in the iframe still, which can be an issue when it's something intensive.

I currently just use Select All then Comment Selection to comment out everything in the JS file, then Build & Run again, but a quick way to replace the current output with basically nothing (or maybe show a little WebAssembly logo) would be useful.