[css-sizing] Define fit-content using clamp()

[Loirooriol]

https://drafts.csswg.org/css-sizing-3/#fit-content-size defines fit-content as

min(max-content size, max(min-content size, stretch-fit size))

At first sight it may not be completely obvious what that formula represents. To avoid this kind of confusions, css-values added clamp(), so why not use it?

clamp(min-content size, stretch-fit size, max-content size)

Seems clearer to me. (Note the desugaring is different, but this doesn't matter because min-content <= max-content)

[Loirooriol]

Could also be used in https://drafts.csswg.org/css-sizing/#valdef-width-fit-content-length-percentage and https://drafts.csswg.org/css-sizing/#valdef-column-width-fit-content-length-percentage