fix: allow AdvancedMarker to accept space-separated multiple class names

[shuuji3]

fix #142

If we want to use classList.add to add multiple class names while keeping the existing class names, we could write like this:

const classNames = className.split(' ')
el.classList.add(...classNames);

But we simply add class names to a brand new div element here so I think we can add the provided className as is.

[usefulthink]

Looks good, thanks!