PR Checks: Check if unknown boxes are present in standard features

Currently, there is an edge case where if a box is not understood by GPAC and also not registered at MP4RA it's not considered as a problem. In fact, this could be the first time this box has been seen (new contribution) so we need to check if it's present in standard features regardless of if it exists in MP4RA or not. This would only apply to `under_consideration` files though.

related #107