A rule that disallows function names not in snake_case.

snake_case is when only lowercase letters are used, and words are separated with underscores, rather than spaces.

For example, this rule considers these function definition valid:

def my_mood do
  :happy
end

defp my_belly do
  :full
end

But it considers these invalid:

def myMood do
  :sad
end

defp myBelly do
  :empty
end