feat: Extract the Item Layout from the List.Item

[alinais]

(Moving from old repo)

Layouts are invisible and are used to place components into a specific arrangement of areas. We currently use the "ItemLayout" component in the ListItem to arrange its parts (media, header, content, etc).

This layout will be used in many other places, such as menus. Stardust will contain a few helpful layouts, of which this is one.

The item layout should be exposed at the top level of the API. We should add a category called "Layouts" just as we currently have "Components". We'll place all invisible Layout components here for now.