Add controlSize/controlProminence modifiers

[carson-katri]

This adds the controlSize and controlProminence modifiers, as well as styles for Button that use them.

I used the default-button appearance which works in Safari for increased prominence buttons, but this isn't supported in other browsers. I can switch it to use custom CSS, but then it may look out of place with other browser buttons...

[MaxDesiatov]

Would it be possible to somehow conditionally use default-button only on Safari and custom CSS in other browsers?

[carson-katri]

I used @supports (-webkit-appearance: default-button) to provide a custom style for browsers that don't support default-button.

[MaxDesiatov]

Seems legit, thanks!