Replace spacing unit with map

[ArmorDarks]

Instead of

$ekzo-spacing-unit: $ekzo-line-height !default;

We can allow to do

$ekzo-spacing-units: (
  base: $ekzo-line-height,
  blog: $ekzo-blog-line-height
) !default;

Yet, all benefits of such change are unclear yet.

[ArmorDarks]

I'm not sure that we need this, because it's anyway impossible to generate anything automatically based on it. So, probably it has no difference with just declaring

$ekzo-spacing-unit: $ekzo-line-height !default;
$blog-spacing-unit: $blog-line-height !default;

We could generate margins and paddings based on it, but it's way too opinionated.