Create dropdown menu component

[DawidKossowski]

📝 Provide a description of the new feature

For now, we use menu components as a part of the menu bar. However, they are incompatible with the rest of the editor and cannot be used as independent components in the toolbar. We have at least two use cases where we need them: AI Commands and Merge tags. This task focuses on creating a new component that resembles menus in the menu bar.

The main requirements are as follows:

1. The main logic should be copied from the current menu bar components. Do not focus too much on refactoring and reusing some commons between the dropdown menu and menu bar components.
2. The components should land in the ckeditor5-ui package as a subfolder of the dropdown component.
3. Add the possibility to implement the search (like in the current AI Commands dropdown).

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