Refactor button properties

[devmount]

We currently use a type property for different button styles, which can be easily confused with the native button type prop.

Proposal:

• Change the type prop to be exactly like the native button type
• Change the variant prop (former type) to reflect the button flavor (primary, secondary, etc)
• Add outline prop (former variant) as boolean if the button is rendered as outline only

This would be much more intuitive for devs.

Originally posted by @devmount in #168 (comment)