Support private instance_variables_to_inspect

[hamajyotan]

This is related to #49.

In ruby/ruby#13555, instance_variables_to_inspect works even when it is defined as a private method.
In fact, the example shown in NEWS defines it as a private method.
https://github.com/ruby/ruby/blob/v4.0.0/NEWS.md?plain=1#L139-L155

I think it would be better for pp to follow this behavior as well.
Below is a minimal reproducible example demonstrating the difference.

require "pp"

class Foo
  def initialize
    @a = "aaa"
    @b = "bbb"
  end

  def instance_variables_to_inspect = [:@b]
end

a = Foo.new
b = Foo.new
b.singleton_class.class_eval { private :instance_variables_to_inspect }

puts RUBY_VERSION
puts a.inspect
puts PP.pp(a, ''.dup)
puts b.inspect
puts PP.pp(b, ''.dup)

#<Foo:0x00007f407f6964c0 @b="bbb">
#<Foo:0x00007f407f6964c0 @b="bbb">
#<Foo:0x00007f407f6961f0 @b="bbb">
#<Foo:0x00007f407f6961f0 @a="aaa", @b="bbb">