Add Gen IV Headbutt encounters (#1520)

* Add Gen IV Headbutt encounters

* Address review

Author: Machine-Maker

data/v2/csv/encounter_condition_prose.csv

@@ -38,3 +38,4 @@ encounter_condition_id,local_language_id,name
 17,9,Friend Safari Slot assignment
 18,9,Great Marsh Daily Slot
 19,9,Honey Tree Group
+20,9,Headbutt Tree Type