Support for flex box gap

[schoettl]

There is the CSS gap property but it's not yet supported in elm-css. Only a gap transition exists already.

Can I add it in a PR?

I think it's very important and handy when working with flexbox.

gap is described here:
https://css-tricks.com/snippets/css/a-guide-to-flexbox/#aa-gap-row-gap-column-gap

Originally posted by @schoettl in #560 (comment)

[schoettl]

For reference:

{-| Not quite correct since CSS gap accepts (slightly) other values.
See https://developer.mozilla.org/en-US/docs/Web/CSS/gap#syntax
-}
gap : LengthOrNumberOrAutoOrNoneOrContent compatible -> Style
gap x =
    property "gap" x.value


gap2 : LengthOrNumberOrAutoOrNoneOrContent compatible -> LengthOrNumberOrAutoOrNoneOrContent compatible -> Style
gap2 x y =
    property "gap" (x.value ++ " " ++ y.value)