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/HTMLTableRowElement.cpp

@@ -65,7 +65,7 @@ JS::NonnullGCPtr<DOM::HTMLCollection> HTMLTableRowElement::cells() const
     if (!m_cells) {
         m_cells = DOM::HTMLCollection::create(const_cast<HTMLTableRowElement&>(*this), DOM::HTMLCollection::Scope::Children, [](Element const& element) {
             return is<HTMLTableCellElement>(element);
-        }).release_value_but_fixme_should_propagate_errors();
+        });
     }
     return *m_cells;
 }