feat(menu): centered variant

A centered variant. I also added the alias center aligned as we have both of this wordings already in other elements.

Especially useful for pagination menu

Author: lubber-de

src/definitions/collections/menu.less

@@ -1476,6 +1476,15 @@ each(@colors, {
   }
 }
 
+& when (@variationMenuCentered) {
+  .ui.center.aligned.menu,
+  .ui.centered.menu {
+    display: inline-flex;
+    transform: translateX(-50%);
+    margin-left:50%;
+  }
+}
+
 & when (@variationMenuInverted) {
   /*--------------
       Inverted

src/themes/default/globals/variation.variables

@@ -267,6 +267,7 @@
 @variationMenuLabeled: true;
 @variationMenuStackable: true;
 @variationMenuFloated: true;
+@variationMenuCentered: true;
 @variationMenuFitted: true;
 @variationMenuBorderless: true;
 @variationMenuCompact: true;