[Experimental][styled-components] Add button primitive

[dratwas]

Description

This is an exploration of x-platform style system. wordpress-mobile/gutenberg-mobile#1411
This is the second iteration on wordpress-mobile/gutenberg-mobile#1386

In this PR I used styled-system with styled-components to create a primitive Button component that could be used in web and mobile and could be styled via props.

• I added A and button components since both of them are used in @components/Button. They accept className as a prop to be backward compatibility ( we need to support already existing themes ) and all style props that are supported by styled-system
• I added the theme provider and theme values (they could be a bit outdated since I took them from my previous PR [Experimental] Cross platform Box component to replace div  #17614 )
• I used created primitives in @components/Button (after that all buttons uses the primitives)
• I passed styles via props instead of CSS in the Inserter button.
• In addition, I created BlockBreadcrumbButton components that use primitives instead of @components/Button just to demonstrate.

How has this been tested?

• Run Gutenberg web
• All buttons should look the same as before
• Check if BlockBreadcrumbButton looks correct with and without hover/focus
• Check if Inserter button looks correct

Screenshots

Types of changes

Experimenting with styled-components and styled-system

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .

[dratwas]

Closing since we use emotion instead