Stub out checkout subflows

[jimbo]

This issue is for the following packages:

[x] venia-concept
[ ] pwa-buildpack
[ ] peregrine
[ ] pwa-module
[ ] pwa-devdocs

This issue is a:

[ ] Bug
[x] Feature suggestion
[ ] Documentation issue
[ ] Other (Please Specify)

Description

Related to #68.

As designed, the checkout workflow consists of three subflows. Future iterations could include more, fewer, or different subflows. Current subflows:

• Shipping address
• Billing address
• Shipping method

A subflow is a workflow that spawns from and returns to the main checkout workflow. The purpose of a subflow is to collect, validate, and save user input. In the current design, subflows operate as modal experiences; control passes from the main workflow to the subflow, then returns to the main workflow when the subflow is complete. In a future design or at an alternate breakpoint, subflows could operate differently; for example, all subflows could be present simultaneously on a single page, and operate on an inline basis rather than a modal one.

Expected result:

Subflow state will be stored in the checkout reducer, and clicking each section of the checkout form will spawn the appropriate subflow. This issue only expects these subflows to exist as stubs; they do not need to contain content or be usable at this point.