Gutenboarding: style composite checkout for gutenboarding flow

[razvanpapadopol]

New checkout screen needs to be styled to be consistent with Gutenboarding experience.
Design issue: #44219

• When accessing checkout from the redesigned launch flow, the purchase button should say Pay and launch.
• When pressing W button or remove the items from the cart, redirect to My Home where launch celebration banner should be visible.

Before

After

Resources:

• Similar update has been made for the old checkout in Frankenflow: Customize checkout step #39447
• Use @automattic/onboarding components if possible.