Allow discovery of existing game files

More of a problem with the Warframe pack rn but I thought having it in general for all games would be better.

Add a functionality to Discord that allows it to look for existing game files in the download destination to prevent having to redownload entire games that you already own just to be able to play them from Discord and instead have Discord hook itself up to the existing game files and run off of those. It shouldn't be that hard to do especially if the game was already in the library as an external platform game (I.e. Discord already knows where the game files are).