Editorial: Return created Realm from InitializeHostDefinedRealm

This AO is not being used in ECMA-262 directly, but from real-world
experience implementing & using is as-is I can say that having to pull
the Realm out of the running execution context is somewhat awkward.

This change is backwards compatible with how the HTML spec uses the AO,
but this would allow simplifying the following:

1. Perform InitializeHostDefinedRealm() with the provided customizations
   for creating the global object and the global this binding.
2. Let realm execution context be the running JavaScript execution
   context.
3. Remove realm execution context from the JavaScript execution context
   stack.
4. Let realm be realm execution context's Realm component.

to

1. Let realm be InitializeHostDefinedRealm() with the provided
   customizations for creating the global object and the global this
   binding.
2. Let realm execution context be the running JavaScript execution
   context.
3. Remove realm execution context from the JavaScript execution context
   stack.

(https://html.spec.whatwg.org/#creating-a-new-javascript-realm)

For hosts that don't have HTML's requirement of removing the running
execution context, it becomes a simple:

1. Let realm be InitializeHostDefinedRealm().

spec.html

@@ -12086,7 +12086,7 @@ <h1>
  </emu-clause>
 
  <emu-clause id="sec-initializehostdefinedrealm" type="abstract operation">
- <h1>InitializeHostDefinedRealm ( ): either a normal completion containing ~unused~ or a throw completion</h1>
+ <h1>InitializeHostDefinedRealm ( ): either a normal completion containing a Realm Record or a throw completion</h1>
  <dl class="header">
  </dl>
 
@@ -12102,7 +12102,7 @@ <h1>InitializeHostDefinedRealm ( ): either a normal completion containing ~unuse
  1. Perform SetRealmGlobalObject(_realm_, _global_, _thisValue_).
  1. Let _globalObj_ be ? SetDefaultGlobalBindings(_realm_).
  1. Create any host-defined global object properties on _globalObj_.
- 1. Return ~unused~.
+ 1. Return realm.
  </emu-alg>
  </emu-clause>