LibJS: Make Heap::allocate<T>() infallible

Stop worrying about tiny OOMs. Work towards #20449.

While going through these, I also changed the function signature in many
places where returning ThrowCompletionOr<T> is no longer necessary.

Userland/Libraries/LibWeb/HTML/HTMLSelectElement.cpp

@@ -42,7 +42,7 @@ JS::GCPtr<HTMLOptionsCollection> const& HTMLSelectElement::options()
             // the select element, and all the option element children of all the optgroup element children
             // of the select element, in tree order.
             return is<HTMLOptionElement>(element);
-        }).release_value_but_fixme_should_propagate_errors();
+        });
     }
     return m_options;
 }