LibJS: Make intrinsics getters return NonnullGCPtr

Some of these are allocated upon initialization of the intrinsics, and
some lazily, but in neither case the getters actually return a nullptr.

This saves us a whole bunch of pointer dereferences (as NonnullGCPtr has
an `operator T&()`), and also has the interesting side effect of forcing
us to explicitly use the FunctionObject& overload of call(), as passing
a NonnullGCPtr is ambigous - it could implicitly be turned into a Value
_or_ a FunctionObject& (so we have to dereference manually).

Userland/Libraries/LibJS/Runtime/ModuleNamespaceObject.cpp

@@ -11,7 +11,7 @@
 namespace JS {
 
 ModuleNamespaceObject::ModuleNamespaceObject(Realm& realm, Module* module, Vector<DeprecatedFlyString> exports)
-    : Object(ConstructWithPrototypeTag::Tag, *realm.intrinsics().object_prototype())
+    : Object(ConstructWithPrototypeTag::Tag, realm.intrinsics().object_prototype())
     , m_module(module)
     , m_exports(move(exports))
 {