Add UI scale mixin support

[vdepizzol]

⚠️ this is a draft PR

This draft includes some considerations for adding support to different UI scales in Primer. Discussion is still happening on https://github.com/github/design-systems/issues/969.

For now this only creates a ui-scale mixin that can be consumed by components that need specialized styles on touch-based devices. It uses pointer: fine/coarse from Media Queries Level 4.

Application example

.dropdown-item {
  display: block;
  padding: $spacer-1 $spacer-3;
  // (...)
  
  @include ui-scale(large) {
    padding-top: $spacer-2;
    padding-bottom: $spacer-2;
  }
}

• Fixes github/design-systems/issues/969

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