Fix required field render bug

[mlhaufe]

For some reason the .required::after class is not being applied. Is this due to some bug with the shadow DOM or is it a bug with the CSSOM insertRule method?

[mlhaufe]

Well, that's irritating. Looks like ::after and ::before pseudos don't work under the shadowRoot (at least in Chromium). I can't find a bug report but there are a couple other people who stumbled upon this problem:

• https://stackoverflow.com/questions/57106412/after-is-not-working-in-shadowroot-web-component-i-am-trying-to-give-some-s
• https://stackoverflow.com/questions/42351806/cannot-add-after-pseudo-element-to-slotted-element-in-shadow-dom

So an extra element will be needed here: <span> *</span>