add submenu-icon part

docs/resources/changelog.md

@@ -9,6 +9,7 @@ New versions of Shoelace are released as-needed and generally occur when a criti
 ## Next
 
 - Added the `checkbox` part and related exported parts to `<sl-tree-item>` so you can target it with CSS [#1318](https://github.com/shoelace-style/shoelace/discussions/1318)
+- Added the `submenu-icon` part to `<sl-menu-item>` (submenus have not been implemented yet, but this part is required to allow customizations)
 - Fixed a bug where changing the size of `<sl-radio-group>` wouldn't update the size of child elements
 - Improved `<sl-button>` so it can accept children of variable heights [#1317](https://github.com/shoelace-style/shoelace/pull/1317)
 - Improved the docs to more clearly explain sizing radios and radio buttons

src/components/menu-item/menu-item.ts

@@ -25,6 +25,7 @@ import type { CSSResultGroup } from 'lit';
  * @csspart prefix - The prefix container.
  * @csspart label - The menu item label.
  * @csspart suffix - The suffix container.
+ * @csspart submenu-icon - The submenu icon, visible only when the menu item has a submenu (not yet implemented).
  */
 @customElement('sl-menu-item')
 export default class SlMenuItem extends ShoelaceElement {
@@ -141,7 +142,7 @@ export default class SlMenuItem extends ShoelaceElement {
 
         <slot name="suffix" part="suffix" class="menu-item__suffix"></slot>
 
-        <span class="menu-item__chevron">
+        <span part="submenu-icon" class="menu-item__chevron">
           <sl-icon name="chevron-right" library="system" aria-hidden="true"></sl-icon>
         </span>
       </div>