Spacing variables

Problem/Motivation

Add some standard spacing root variables, similar to the Drupal Core setup seen below, and update all existing .pcss files with these new variables.

/**
   * Spaces.
   */
  --space-xl: 3rem; /* 3 * 16px = 48px */
  --space-l: 1.5rem; /* 1.5 * 16px = 24px */
  --space-m: 1rem; /* 1 * 16px = 16px */
  --space-s: 0.75rem; /* 0.75 * 16px = 12px */
  --space-xs: 0.5rem; /* 0.5 * 16px = 8px */