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/GeneratorFunctionConstructor.cpp

@@ -12,7 +12,7 @@
 namespace JS {
 
 GeneratorFunctionConstructor::GeneratorFunctionConstructor(Realm& realm)
-    : NativeFunction(static_cast<Object&>(*realm.intrinsics().function_constructor()))
+    : NativeFunction(static_cast<Object&>(realm.intrinsics().function_constructor()))
 {
 }