LibWeb: Hook up the HostInitializeShadowRealm callback

This is enough for a basic shadow realm to work :^)

There is more that we still need to implement here such as module
loading and fixing up the global object, but this is enough to get some
basic usage working.

Tests/LibWeb/Text/expected/HTML/shadow-realm-async-evaluate.txt

@@ -0,0 +1 @@
+PASS: Exception thrown 'TypeError: Wrapped value must be primitive or a function object, got [object Promise]'

Tests/LibWeb/Text/expected/HTML/shadow-realm-sync-evaluate.txt

@@ -0,0 +1 @@
+Shadow realm evaluation returned: 2

Tests/LibWeb/Text/input/HTML/shadow-realm-async-evaluate.html

@@ -0,0 +1,13 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const realm = new ShadowRealm()
+        const aSync = realm.evaluate(`async () => 1 + 1`);
+        try {
+            aSync();
+            println('Fail! No exception thrown');
+        } catch (e) {
+            println(`PASS: Exception thrown '${e}'`);
+        }
+    })
+</script>

Tests/LibWeb/Text/input/HTML/shadow-realm-sync-evaluate.html

@@ -0,0 +1,8 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const realm = new ShadowRealm();
+        const result = realm.evaluate(`() => 1 + 1`)();
+        println(`Shadow realm evaluation returned: ${result}`);
+    });
+</script>

Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp

@@ -15,11 +15,13 @@
 #include <LibJS/Runtime/FinalizationRegistry.h>
 #include <LibJS/Runtime/ModuleRequest.h>
 #include <LibJS/Runtime/NativeFunction.h>
+#include <LibJS/Runtime/ShadowRealm.h>
 #include <LibJS/Runtime/VM.h>
 #include <LibJS/SourceTextModule.h>
 #include <LibWeb/Bindings/ExceptionOrUtils.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/Bindings/SyntheticHostDefined.h>
 #include <LibWeb/Bindings/WindowExposedInterfaces.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/MutationType.h>
@@ -35,7 +37,9 @@
 #include <LibWeb/HTML/Scripting/Fetching.h>
 #include <LibWeb/HTML/Scripting/ModuleScript.h>
 #include <LibWeb/HTML/Scripting/Script.h>
+#include <LibWeb/HTML/Scripting/SyntheticRealmSettings.h>
 #include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
+#include <LibWeb/HTML/ShadowRealmGlobalScope.h>
 #include <LibWeb/HTML/TagNames.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HTML/WindowProxy.h>
@@ -560,6 +564,36 @@ ErrorOr<void> initialize_main_thread_vm(HTML::EventLoop::Type type)
         HTML::fetch_single_imported_module_script(*module_map_realm, url.release_value(), *fetch_client, destination, fetch_options, *module_map_realm, fetch_referrer, module_request, perform_fetch, on_single_fetch_complete);
     };
 
+    // https://whatpr.org/html/9893/webappapis.html#hostinitializeshadowrealm(realm,-context,-o)
+    // 8.1.6.8 HostInitializeShadowRealm(realm, context, O)
+    s_main_thread_vm->host_initialize_shadow_realm = [](JS::Realm& realm, NonnullOwnPtr<JS::ExecutionContext> context, JS::ShadowRealm& object) -> JS::ThrowCompletionOr<JS::NonnullGCPtr<JS::Object>> {
+        // FIXME: 1. Set realm's is global prototype chain mutable to true.
+
+        // 2. Let globalObject be a new ShadowRealmGlobalScope object with realm.
+        auto global_object = HTML::ShadowRealmGlobalScope::create(realm);
+
+        // 3. Let settings be a new synthetic realm settings object that this algorithm will subsequently initialize.
+        auto settings = HTML::SyntheticRealmSettings {
+            // 4. Set settings's execution context to context.
+            .execution_context = move(context),
+
+            // 5. Set settings's principal realm to O's associated realm
+            .principal_realm = object.shape().realm(),
+
+            // 6. Set settings's underlying realm to realm.
+            .underlying_realm = realm,
+
+            // 7. Set settings's module map to a new module map, initially empty.
+            .module_map = realm.heap().allocate<HTML::ModuleMap>(realm),
+        };
+
+        // 8. Set realm.[[HostDefined]] to settings.
+        realm.set_host_defined(make<Bindings::SyntheticHostDefined>(move(settings)));
+
+        // 9. Return globalObject.
+        return global_object;
+    };
+
     s_main_thread_vm->host_unrecognized_date_string = [](StringView date) {
         dbgln("Unable to parse date string: \"{}\"", date);
     };