getFolderPrefix
Extension function to get the folder prefix based on the TvType. Non-episode-based types will return a base folder name, while episode-based types will have their files placed in subfolders using a sanitized title name.
For the actual folder path, refer to ResultViewModel2().getFolder(), which combines the folder prefix and, if necessary, the sanitized name to a sub-folder. The folder prefix will be used in the root directory of the configured downloads directory.
Return
the folder prefix corresponding to the TvType, which is used as the root directory.