[css-sizing][naming] Renaming fill-available to fill

Thinking about the sizing behavior of 'fill-available', Tab and I think
it would be better to just name it 'fill'. It doesn't really take into
account the "available" space in the sense that it doesn't account for
any other content in the box. So it would probably be less surprising
to authors to name this keyword just 'fill'.

Thoughts?

~fantasai