Add "items-start" support to Stack

app/soapbox/components/ui/stack/stack.tsx

@@ -21,13 +21,14 @@ const justifyContentOptions = {
 
 const alignItemsOptions = {
   center: 'items-center',
+  start: 'items-start',
 };
 
 interface IStack extends React.HTMLAttributes<HTMLDivElement> {
   /** Size of the gap between elements. */
   space?: SIZES,
   /** Horizontal alignment of children. */
-  alignItems?: 'center',
+  alignItems?: 'center' | 'start',
   /** Vertical alignment of children. */
   justifyContent?: 'center',
   /** Extra class names on the <div> element. */