[V4-beta.9] Add directions for properties like margin

[stijns96]

I don't always want variables or values to be added to all spacings, but only to values like margin or padding. E.g. we add container spacings to margin that can be used as mt, m-y and m-b.

Proposal

--margin-*: value; // top, right, bottom, left, x, y
--margin-x: value; // right, left, x
--margin-y: value; // top, bottom, y

--margin-{top,right,bottom,left} are quite obvious.

[stijns96]

Kind of related #15651

Still like to see some opinions about it.