Export slot's classNames as static fields from the Component

[mnajdova]

Feature Request

Problem description

For some of the components' slots that does not have dedicated components, we are defining classNames in order for defining css selectors for them. As an example, in the MenuItem component, we have custom class for the wrapper element: ui-menu__item__wrapper which is not exported anywhere from stardust. In addition to this, we should make sure that all slot's inside the components can be safely selected for applying styles.

Proposed solution

Let's add the static slotClassNames object to the components that are defining some custom classNames for the slots (See the example in the ChatMessage component as a guidance).

MVP