[w3c/manifest] Add offline_url option (#774)

A proposal for a new, optional property for manifest files: `offline_url`. This should point to a URL like `start_url`. When present, if a PWA would serve the default browser offline page (like the Chrome offline dinosaur), it would instead serve the `offline_url` page. This should _not_ override the ability of Service Workers to serve pages when offline, it would _only_ be served if a URL isn't covered by a Service Worker and the browser would need inform users that they are offline.

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https://github.com/w3c/manifest/issues/774