[DuckTypist] target priorities

[fabi1cazenave]

Until #189 there was no letter priority in DuckTypist. Now the two latest letters are prioritized, but this might not be the best choice for everyone — e.g. learning ENTR in Ergo‑L can remain tricky even if 12 or more letters have already been unlocked.

A possible fix would be to propose three options;

• prioritize the two latest letters
• prioritize the two most frequently mistyped letters
• no priority at all, just propose words with the unlocked letters