Usability improvement: Generate the custom Jumpstart pack file at runtime by iterating over a directory of .dck files.

The existing method requiring users to manually collate all decks into a single .txt file could be made much easier by generating the pack file at runtime by iterating over a directory of XMage .dck files. This also makes creating and editing the individual decks much easier as they can be simply edited in the XMage deck editor.

Additionally, to simplify deck testing allow the .dck files to have cards in the sideboard but ignore them when creating the pack list.