folder file parser should be case insensitive

[pokepress]

I kept running into an issue with this line in data_utils.py:

assert input_name in input_paths, f'{input_name} is not in {input_key}_paths.'

Because my HR and LR files had different casings on their extensions. Windows considers them equivalent (and changing it is more complicated than just editing the filename in Windows Explorer, as it will change the capitalization back), so I changed the code to:

assert input_name.casefold() in map(str.casefold, input_paths), f'{input_name} is not in {input_key}_paths.'

But there's probably a cleaner/more appropriate way to achieve this.