Difficulty Death Scaler

Change difficulty after 7 deaths:

• reduce maximal number of hearts before disabling natural regeneration (by @Leo21)
• disable natural regeneration for 20 deaths and more (by @Leo21)

Fix: difficulty not set to Difficulty Death Scaler's default difficulty. (by @anhgelus)

Update yarn mappings and clean the code. (by @Leo21 and @anhgelus)