Allow Custom Dungeons to overwrite equipment lab text + Custom Dungeon Music in Equipment Lab

I originally wanted to keep this feature as a part of a mod until I realized that shopkeeper mods likely completely overhaul the equipment lab code to replace the text. Its probably better for this feature to be in vanilla so those shopkeeper mods can be updated to remove potentially outdated Equipment Lab code from floating around, terrorizing hosts. This commit rawsets the original textdata table to simplify the process for Shopkeeper mods to edit it, and also makes the equipment lab check for a new custom dungeon table before assigning anything to the, now empty, local textdata table. There's also a second commit that allows custom dungeon defined shop music to play in the equipment lab. It checks for a equiplab_music variable in the case dungeon makers want separate music from the shop, but falls back onto the custom dungeon's shop music if that isnt defined, then falls back onto the normal SHOP lump if that isnt defined either.