gems: replace pry with debug.

Replace the 'pry' gem with the 'debug' gem. The debug gem is the new
standard debugger in Ruby 3.1.

Use `binding.break` instead of `binding.pry` to set breakpoints.

Install the extension below for VS Code support.

https://github.com/ruby/debug#how-to-use
https://marketplace.visualstudio.com/items?itemName=KoichiSasada.vscode-rdbg

bin/rdbg

@@ -4,15 +4,13 @@
 #
 # This file was generated by Bundler.
 #
-# The application 'pry' is installed as part of a gem, and
+# The application 'rdbg' is installed as part of a gem, and
 # this file is here to facilitate running it.
 #
 
-require "pathname"
-ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
 
-bundle_binstub = File.expand_path("../bundle", __FILE__)
+bundle_binstub = File.expand_path("bundle", __dir__)
 
 if File.file?(bundle_binstub)
   if File.read(bundle_binstub, 300) =~ /This file was generated by Bundler/
@@ -26,4 +24,4 @@ end
 require "rubygems"
 require "bundler/setup"
 
-load Gem.bin_path("pry", "pry")
+load Gem.bin_path("debug", "rdbg")