Add an exercise that shows caches are origin-wide

…and shows how you'd allow an old serviceworker to use old files, while the new one prepares the new version.

Could be something as simple as changing the h1 color in the CSS file.

This would need to do before the introduction of `skipWaiting`.

Start by reading the existing tutorial exercise sequence and identify where this exercise should appear before the introduction of `skipWaiting`. Define the exercise around an old service worker using old files while a new version prepares new files, using an h1 color change in the CSS as the visible result.

Written by the indexing model from the issue text.