#3349298 New Checkout: split layout (take 2)

Originally attempted in #41063 and reverted in #41363

This does a few things with the ultimate goal of splitting out a summary into a separate column on desktop screens.

• adds the floated CheckoutSummary component to composite-checkout (package, demo, documentation)
• add CheckoutStepArea component to handle styling of CheckoutSteps + CheckoutStepBody
• uses CheckoutSummary in WPcom’s implementation of composite-checkout
• moves the current site’s address to the review step
• adds a default set of purchase features to the summary (per-product features coming in a future PR)
• hides CheckoutSummary on mobile screens (collapse mobile toggle coming in future PR)

Before	After

Fixes: #41088

To test:

• it might help to step through the PR, commit by commit, but no promises that I didn’t change my mind later on in the PR
• run the composite-checkout demo with yarn run composite-checkout-demo
• verify that minimal CheckoutSummary is displayed
• verify that the demo otherwise works as expected
• from dev or calypso.live, and a plan and/or domain to your cart and visit Checkout with ?flags=composite-checkout-testing
• verify that on desktop (>960px), the checkout summary is displayed, floated to the right, with a list of default features, and the tax (once calculated) and total
• verify that on screens smaller than desktop, the checkout summary is hidden
• verify that Checkout otherwise works as expected