[Accessibility] Detach heading size from the semantic element

[andresgalante]

The Heading element has a size prop.

This size changes the type of heading the element is, from h1 to h4.

Cosmos can't assume to know the author's DOM tree, and in order to have good accessibility landmarks and good heading order are crucial.

To do:

• Create a new prop to set the heading type detached from the size and styles.
• Write docs to explain how landmarks work.