LibJS: Bring the Array constructor slightly closer to the specification

Specifically, we now cast to a u32 instead of an i32, as well as use
the validity check required by the specification. The current
constructor is still quite far from the specification, as we directly
set the indexed properties' length instead of going through the Array's
overriden DefineOwnProperty. (and as a result the checks imposed by the
ArraySetLength abstract operation)

Userland/Libraries/LibJS/Runtime/ArrayConstructor.cpp

@@ -51,13 +51,14 @@ Value ArrayConstructor::call()
  return Array::create(global_object());
 
  if (vm().argument_count() == 1 && vm().argument(0).is_number()) {
- auto array_length_value = vm().argument(0);
- if (!array_length_value.is_integral_number() || array_length_value.as_i32() < 0) {
+ auto length = vm().argument(0);
+ auto int_length = length.to_u32(global_object());
+ if (int_length != length.as_double()) {
  vm().throw_exception<RangeError>(global_object(), ErrorType::InvalidLength, "array");
  return {};
  }
  auto* array = Array::create(global_object());
- array->indexed_properties().set_array_like_size(array_length_value.as_i32());
+ array->indexed_properties().set_array_like_size(int_length);
  return array;
  }