Allow buttons to accept inline children

The button block currently doesn't allow for child elements. I understand the reasoning: only inline elements can be children of `<button>`, so maybe it's easier just to prevent any children.

However, a common use case is when you want to create a button with an icon/image on it. Right now, to do that you need to create a box, set the HTML tag to `button` (which is buggy in its own way: https://forum.builder.io/t/blocks-dont-respect-the-background-color-when-you-set-the-html-tag-to-button/2657/2), and then add your children.

Would be much simpler if you could add a child element and then Visual Editor highlights an error if the element isn't inline, prompting the user to set `display: inline-block` or something like that.