dataset_vocab - tokenizer_vocab
{' ', 'à', 'ç', 'è', 'ë', 'í', 'ï', 'ö', 'ü'}

To handle the unsupported characters identified in the previous step, define a function that maps these characters to 
valid tokens. Note that spaces are already replaced by ▁ in the tokenizer and don't need to be handled separately.