Flag ServiceWorkerGlobalScope as SecureContext

Context for this is Open Web Docs people looking into automating the handling
of "this feature is available only in secure contexts" banners in MDN pages.
This led to the discussion in:
w3c/webref#1142 (comment)

When it is set, the `[SecureContext]` IDL extended attribute explicitly gives
the information. That said, to avoid redundancies, that attribute is not set on
interfaces that are exposed (through `[Exposed=xxx]`) on globals that are
already restricted to secure contexts/

The Service workers spec is clear that service workers must execute in secure
contexts:
https://w3c.github.io/ServiceWorker/#secure-context

However, it does not fully say so in the IDL itself. More specifically, when
an interface defined in another spec has `[Exposed=ServiceWorker]`, that's a
reference to the `ServiceWorkerGlobalScope` interface, and that interface does
not have a `[SecureContext]` attribute.

This commit adds the `[SecureContext]` attribute to `ServiceWorkerGlobalScope`.

This approach is consistent with the way `WorkletGlobalScope`, from which a
number of other globals inherit, is defined:
https://html.spec.whatwg.org/multipage/worklets.html#worklets-global

docs/index.bs

@@ -1060,7 +1060,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
     <h3 id="serviceworkerglobalscope-interface">{{ServiceWorkerGlobalScope}}</h3>
 
     <pre class="idl">
-      [Global=(Worker,ServiceWorker), Exposed=ServiceWorker]
+      [Global=(Worker,ServiceWorker), Exposed=ServiceWorker, SecureContext]
       interface ServiceWorkerGlobalScope : WorkerGlobalScope {
         [SameObject] readonly attribute Clients clients;
         [SameObject] readonly attribute ServiceWorkerRegistration registration;