Update web sample cart demo

[kieran-osgood-shopify]

Summary

Expanding the capabilities of the web demo to demonstrate its features more completely

The UI Reorganization was based on the columns:
Col 1: Settings / setup
Col 2: Interactive demo
Col 3: Debugging Events + Component state (Component state previously lived in Col 2)

Before	After
Product card gave confusing UX (variant selector does nothing, the input is a checkout url)	Reorganised UX with hints to users to lead people in the right direction on use
Only allowed opening checkout with a preconfigured checkout url	Adds features to allow the creation of multi item carts based on the products.json

Demo

See this ticket: https://github.com/shop/issues-checkout-kit/issues/1140

Test plan

• shadowenv exec -- /opt/dev/bin/dev web test
• shadowenv exec -- /opt/dev/bin/dev web lint
• shadowenv exec -- /opt/dev/bin/dev web sample build
• shadowenv exec -- /opt/dev/bin/dev web build
• shadowenv exec -- /opt/dev/bin/dev web verify

[kieran-osgood-shopify]

• feat: add resizable columns #436
• Update web sample cart demo #420 👈 (View in Graphite)
• main

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