Add Type Safety For Length Units In Sizes

[JamieB-gu]

Decide whether to introduce a more sophisticated type for a size value, which will allow us to avoid some usages of calc by calculating some values server-side. For example:

const enum Length {
    Px,
    Vw,
    Percentage,
}

type SizeValue = {
    unit: Length;
    value: number;
}

type Size = {
    breakpoint: Breakpoint;
    size: SizeValue;
}

[mxdvl]

Sorry… this need to stay open as it’s for upcoming work.