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/Intl/CollatorCompareFunction.cpp

@@ -17,7 +17,7 @@ NonnullGCPtr<CollatorCompareFunction> CollatorCompareFunction::create(Realm& rea
 }
 
 CollatorCompareFunction::CollatorCompareFunction(Realm& realm, Collator& collator)
-    : NativeFunction(*realm.intrinsics().function_prototype())
+    : NativeFunction(realm.intrinsics().function_prototype())
     , m_collator(collator)
 {
 }