[w3c/ServiceWorker] Expose `display` mode (e.g. `standalone`) on `client` (#1370)

Per a recent partner conversation, it can be useful at fetch time to understand the display mode of the client making a `fetch`. In keeping with our principle of telling developers what we know which they could already get in an inelegant way, we should perhaps expose this on Clients.

It's very inelegant to get this data from the Service Worker context today; it's only available via DOM through a media query, and propagating it through to a fetch is difficult.

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