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/LibWeb/CSS/CSSImportRule.cpp

@@ -25,7 +25,7 @@ JS_DEFINE_ALLOCATOR(CSSImportRule);
 JS::NonnullGCPtr<CSSImportRule> CSSImportRule::create(URL::URL url, DOM::Document& document)
 {
     auto& realm = document.realm();
-    return realm.heap().allocate<CSSImportRule>(realm, move(url), document);
+    return realm.create<CSSImportRule>(move(url), document);
 }
 
 CSSImportRule::CSSImportRule(URL::URL url, DOM::Document& document)