Native support for radius 0

[marcopiii]

Currently, the theme supports the radius 4, 8, and 16.
If you need a custom border-radius in your project you can easily extend the native sprinkles to introduce the 0 case.
However, it seems a reasonably typical case so maybe it should be supported by the native theme.

[gabro]

@Phyele what do you think? I think it's fair to allow 0 as a radius value for all Bento projects, but I wanted to confirm this with you

[gabro]

discussed offline with @Phyele, it's correct that Bento supports it, so I'll add it