feat(AvatarStack): Add support for className prop (#5097)

* add support for className prop in the avatar stack

* add changelog

* fix className location

.changeset/angry-brooms-knock.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': minor
+---
+
+Added className prop to the AvatarStack component

packages/react/src/AvatarStack/AvatarStack.tsx

@@ -183,17 +183,28 @@ export type AvatarStackProps = {
   alignRight?: boolean
   disableExpand?: boolean
   size?: number | ResponsiveValue<number>
+  className?: string
   children: React.ReactNode
 } & SxProp
 
-const AvatarStack = ({children, alignRight, disableExpand, size, sx: sxProp = defaultSxProp}: AvatarStackProps) => {
+const AvatarStack = ({
+  children,
+  alignRight,
+  disableExpand,
+  size,
+  className,
+  sx: sxProp = defaultSxProp,
+}: AvatarStackProps) => {
   const count = React.Children.count(children)
-  const wrapperClassNames = clsx({
-    'pc-AvatarStack--two': count === 2,
-    'pc-AvatarStack--three': count === 3,
-    'pc-AvatarStack--three-plus': count > 3,
-    'pc-AvatarStack--right': alignRight,
-  })
+  const wrapperClassNames = clsx(
+    {
+      'pc-AvatarStack--two': count === 2,
+      'pc-AvatarStack--three': count === 3,
+      'pc-AvatarStack--three-plus': count > 3,
+      'pc-AvatarStack--right': alignRight,
+    },
+    className,
+  )
   const bodyClassNames = clsx('pc-AvatarStackBody', {
     'pc-AvatarStack--disableExpand': disableExpand,
   })