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/Encoding/TextEncoder.cpp

@@ -16,7 +16,7 @@ JS_DEFINE_ALLOCATOR(TextEncoder);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<TextEncoder>> TextEncoder::construct_impl(JS::Realm& realm)
 {
-    return realm.heap().allocate<TextEncoder>(realm, realm);
+    return realm.create<TextEncoder>(realm);
 }
 
 TextEncoder::TextEncoder(JS::Realm& realm)