Add fontSize prop to Separator

[mturoci]

via @vopani

[lo5]

One of the drawbacks of allowing font size changes to the separator is that doing so would hamper consistency of look and feel.

That said, this ask might be because the separator does a better job of visually separating a form's sections (compared to ui.text_*()) because of the existence of the horizontal line across the text. If so, instead of allowing arbitrary font sizes, we should probably use preset heading levels (see size argument to ui.text()).

cc @vopani