Enhancement: make Style/GuardClause aware of dynamic methods

[vlad-pisanov]

Currently, Style/GuardClause only looks at statically-defined methods, and skips those defined via define_method or define_singleton_method:

def foo
  if cond
    42   # Style/GuardClause flagged ✔️
  end
end

define_method(:bar) do
  if cond
    42   # Style/GuardClause ignores 😔
  end
end