Feature: add support for parsing viewport units

[kristerkari]

Calculating styles dynamically makes it possible to add support for viewport units.

• Viewport Width (vw) – A percentage of the full viewport width. 10vw will resolve to 10% of the current viewport width, or 48px on a phone that is 480px wide. The difference between % and vw is most similar to the difference between em and rem. A % length is relative to local context (containing element) width, while a vw length is relative to the full width of the browser window.
• Viewport Height (vh) – A percentage of the full viewport height. 10vh will resolve to 10% of the current viewport height.
• Viewport Minimum (vmin) – A percentage of the viewport width or height, whichever is smaller. 10vmin will resolve to 10% of the current viewport width in portrait orientations, and 10% of the viewport height on landscape orientations.
• Viewport Maximum (vmax) – A percentage of the viewport width or height, whichever is larger. 10vmax will resolve to 10% of the current viewport height in portrait orientations, and 10% of the viewport width on landscape orientations. Sadly, and strangely, vmax units are not yet available on Internet Explorer or Edge.

The way that this would work is that the values would be parsed with units and a flag __viewportUnits: true would be enabled. Those values would then during runtime be compared agains React Native's viewport and transformed to pixel values.

More info:
https://css-tricks.com/fun-viewport-units/

[kristerkari]

Currently waiting for this:
styled-components/css-to-react-native#83