LibJS+LibWeb: Use realm.create<T> instead of heap.allocate<T>

The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.

As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.

Libraries/LibTest/JavaScriptTestRunner.h

@@ -284,7 +284,7 @@ inline JSFileResult TestRunner::run_file_test(ByteString const& test_path)
         *g_vm,
         [&](JS::Realm& realm_) -> JS::GlobalObject* {
             realm = &realm_;
-            global_object = g_vm->heap().allocate<TestRunnerGlobalObject>(*realm, *realm);
+            global_object = realm->create<TestRunnerGlobalObject>(*realm);
             return global_object;
         },
         nullptr));