Require shellwords where it is used

Ruby 3.1 doesn't require this by default anymore.

activerecord/Rakefile

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "shellwords"
 require "rake/testtask"
 
 require_relative "test/config"

activestorage/lib/active_storage/previewer/video_previewer.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "shellwords"
+
 module ActiveStorage
   class Previewer::VideoPreviewer < Previewer
     class << self

railties/Rakefile

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "shellwords"
 require "rake/testtask"
 
 task default: :test

railties/test/isolation/abstract_unit.rb

@@ -9,6 +9,7 @@
 # It is also good to know what is the bare minimum to get
 # Rails booted up.
 require "fileutils"
+require "shellwords"
 
 require "bundler/setup" unless defined?(Bundler)
 require "active_support"