[Ruby] S1172 does not work when the file has more than one function

The existing rule UnusedFunctionParameter S1172 does not work for Ruby code.

The rule raises on methods that are either:

• under TopLevelTree (not in a class) - but for ruby, the parent of the context of a top-level function is a Block, not a TopLevelTree (except when the function is the only piece of code in the file)
• a private class method, but in ruby the modifier doesn't seem to get in the tree

def is_not_used( a )
  puts "Hello, World!" # FN
end

class Foo
    private def is_not_used( a )
      puts "Hello, World!" # FN
    end
end

class Bar

    private

    def is_not_used( a )
      puts "Hello, World!" # FN
    end
end

Note: the rule works only if the function is the only piece of code in the file

def foo(unused)
  puts 'Hello' # Noncompliant
end