Add require 'set' for Ruby < 3.2 compatibility

Set class became a core class in Ruby 3.2, but earlier versions
(2.7, 3.0, 3.1) require explicit 'require "set"' to use it.
This fixes NameError: uninitialized constant IRB::Set.

Author: ahogappa

lib/irb/init.rb

@@ -4,6 +4,8 @@
 #   	by Keiju ISHITSUKA(keiju@ruby-lang.org)
 #
 
+require 'set'
+
 module IRB # :nodoc:
   @CONF = {}
   @INITIALIZED = false