[RNMobile][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 can be used in web and mobile and can be styled via props. That PR contains the mobile part.

• I added the button primitive component (TouchableOpacity) that can be used with styled-system
• I added the theme provider and theme values (they can be a bit outdated since I took them from my previous PR [Experimental] Cross platform Box component to replace div  #17614 )
• I used created primitive in @components/Button
• I added isSmall, isLarge and isPrimary props and set button style related to them (just to demonstrate)
• I used Button primitive in breadcrumbs
• I set isLarge and isPrimary to @components/Button in ButtonBlockAppender component to demonstrate they work :)

How has this been tested?

• Run gutenberg-mobile app
• All buttons should look the same as before
• Check if breadcrumbs are rendered properly
• Check if block appender has blue background and additional padding (because of isLarge and isPrimary)

Screenshots

without isLarge and isPrimary

with isLarge and isPrimary

Types of changes

Experimenting with styled-components and styled-system

[dratwas]

Closing since we use emotion