Bug 1826833 - add design tokens for moz-button-group r=jules,desktop-theme-reviewers,dao,reusable-components-reviewers

This patch introduces a simple spacing token for use in `moz-button-group`.

Differential Revision: https://phabricator.services.mozilla.com/D180697

toolkit/content/widgets/moz-button-group/moz-button-group.css

@@ -12,5 +12,5 @@
 }
 
 ::slotted(button:not(:first-child, .popup-notification-dropmarker)) {
-  margin-inline-start: 8px !important;
+  margin-inline-start: var(--spacing-small) !important;
 }

toolkit/themes/shared/design-system/tokens-shared.css

@@ -94,6 +94,9 @@
   /** Size **/
   --size-item-small: 16px;
   --size-item-large: 32px;
+
+  /** Spacing **/
+  --spacing-small: 0.5rem;
 }
 
 @media not (prefers-contrast) {