[e.cash] Add Button Component

Summary: adding a flexible button component to be used throughout

Test Plan: preview and check the hero section. Can try passing the variant prop white or outline to see the other styles

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18323