Add BTL_initSkillChange and BTL_changeSkillSelect to Battle.LUA: Non-explosive edition

Apparently the web IDE is cursed and doesn't understand how to make a proper commit, so I extracted the changes made in !48 (closed) and rehosted them here so the changes can be viewed(R.E.D agreed to let me do this).

This allows skills to be changed into other skills (instead of editing the attackDefs of the skill, which causes desynchs).

Authored by: @redmetalsonic