* Parse an `offset` string to extrapolate `x` and `y` numeric offsets. * @function * @memberof {modifiers~offset} * @private * @argument {String} offset * @argument {Object} popperOffsets * @argument {Object} referenceOffsets * @argument {String} basePlacement * @returns {Array} a two cells
(offset, popperOffsets, referenceOffsets, basePlacement)