Controlling difficulty

# Suggestion1

We make an assumption that whether I mistype depends on the current character and the 2-3 previous ones (not more).

One can just create an `window_size + 1` dimensional array where each dimension has size `vocab_size` that stores the errors somehow.

No file, test, or entry point is named. Start by locating the current difficulty or error model and how it represents character history; the issue does not define implementation or validation criteria, so completion would require agreeing on those details first.

Written by the indexing model from the issue text.