Playground Block: Editor script mode breaks the preview

[adamziel]

@ironnysh wrote:

The only problem I had was going from Plugin to Editor script mode: adding and running code in Plugin mode worked every time, but Editor script "broke" the preview without providing a meaningful error.

The Editor script mode isn't a critical feature so I'm going to hide the select control for now to make the block suitable for Learn.wp.org. Let's fix it once a related feature requests comes in.