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

@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+#
+# This file was generated by Bundler.
+#
+# The application 'rdbg' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
+
+bundle_binstub = File.expand_path("bundle", __dir__)
+
+if File.file?(bundle_binstub)
+  if File.read(bundle_binstub, 300) =~ /This file was generated by Bundler/
+    load(bundle_binstub)
+  else
+    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
+Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
+  end
+end
+
+require "rubygems"
+require "bundler/setup"
+
+load Gem.bin_path("debug", "rdbg")