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/HTMLTableSectionElement.cpp

@@ -41,7 +41,7 @@ JS::NonnullGCPtr<DOM::HTMLCollection> HTMLTableSectionElement::rows() const
     if (!m_rows) {
         m_rows = DOM::HTMLCollection::create(const_cast<HTMLTableSectionElement&>(*this), DOM::HTMLCollection::Scope::Children, [](Element const& element) {
             return is<HTMLTableRowElement>(element);
-        }).release_value_but_fixme_should_propagate_errors();
+        });
     }
     return *m_rows;
 }