Specify a `Size` without unit (e.g. for `line-height`)

[vyorkin]

It would be nice to be able to specify a number without a unit for Size Value.
I mean, does it makes sense to make a PR with such function?

num ∷ Number -> Size Value
num = Size <<< value

So it would be possible to write:

lineHeight $ num 2.0

Not sure if this looks good though 😂

[vyorkin]

In Clay its named unitless: https://github.com/sebastiaanvisser/clay/blob/03f90289eba89fa72cfc81b1f9685f699c954fd9/src/Clay/Size.hs#L123