DEV: implements <DropdownMenu /> (#26917)

DropdownMenu component is meant as a way to describe the content of menus.

Syntax:

```
<DropdownMenu as |dm|>
  <dm.item class="test">
    First
  </dm.item>
  <dm.divider class="foo" />
  <dm.item class="bar">
    Second
  </dm.item>
</DropdownMenu>
```

app/assets/javascripts/float-kit/addon/components/d-menu.gjs

@@ -99,6 +99,8 @@ export default class DMenu extends Component {
             (concat this.options.identifier "-content")
           }}
           @inline={{(isTesting)}}
+          data-identifier={{@instance.options.identifier}}
+          data-content
         >
           {{#if (has-block)}}
             {{yield this.componentArgs}}