Add offset-none to the responsive offset loop

[thisisdano]

Thanks to @robertromore for bringing this up in #3013

We'd omitted offset-none from the responsive offsets loop. Since none isn't one of the keys in $system-layout-grid-widths, we have to add it to the loop manually, as we do in

uswds/src/stylesheets/core/_layout-grid.scss

Line 133 in a7cbda2

.#{$namespace-grid}offset-none {

faulty loop:

// responsive offsets
@each $mq-key, $mq-value in $system-breakpoints {
  @if map-get($theme-utility-breakpoints, $mq-key) {
    @each $width-key, $width-value in $system-layout-grid-widths {
      @include at-media($mq-key) {
        .#{$mq-key}\:#{$namespace-grid}offset-#{$width-key} {
          $props: append-important($grid-global, $width-key);
          @include grid-offset($props);
        }
      }
    }
// --> need to add the offset-none rule here.
  }
}

updated loop:

// responsive offsets
@each $mq-key, $mq-value in $system-breakpoints {
  @if map-get($theme-utility-breakpoints, $mq-key) {
    @each $width-key, $width-value in $system-layout-grid-widths {
      @include at-media($mq-key) {
        .#{$mq-key}\:#{$namespace-grid}offset-#{$width-key} {
          $props: append-important($grid-global, $width-key);
          @include grid-offset($props);
        }
      }
    }
    @include at-media($mq-key) {
      .#{$mq-key}\:#{$namespace-grid}offset-none {
        $props: append-important($grid-global, none);
        @include grid-offset($props);
      }
    }
  }
}