Improve error message for missing listen gem

A user reported seeing the following error when trying to use the
EventedFileUpdateChecker in their app:

```shell
$ rails server
=> Booting Puma
=> Rails 7.0.4 application starting in development
=> Run `bin/rails server --help` for more startup options
Exiting
/usr/local/lib/ruby/gems/3.0.0/gems/zeitwerk-2.6.6/lib/zeitwerk/kernel.rb:38:in `require': cannot load such file -- listen (LoadError)
	from /usr/local/lib/ruby/gems/3.0.0/gems/zeitwerk-2.6.6/lib/zeitwerk/kernel.rb:38:in `require'
	from /usr/local/lib/ruby/gems/3.0.0/gems/activesupport-7.0.4/lib/active_support/evented_file_update_checker.rb:6:in `<top (required)>'
	from /usr/local/lib/ruby/gems/3.0.0/gems/zeitwerk-2.6.6/lib/zeitwerk/kernel.rb:38:in `require'
	from /usr/local/lib/ruby/gems/3.0.0/gems/zeitwerk-2.6.6/lib/zeitwerk/kernel.rb:38:in `require'
	from /home/ross/Data/EmySystem/newentry/Entry2/config/environments/development.rb:84:in `block in <top (required)>'
	from /usr/local/lib/ruby/gems/3.0.0/gems/railties-7.0.4/lib/rails/railtie.rb:257:in `instance_eval'
	from /usr/local/lib/ruby/gems/3.0.0/gems/railties-7.0.4/lib/rails/railtie.rb:257:in `configure'
	from /home/ross/Data/EmySystem/newentry/Entry2/config/environments/development.rb:3:in `<top (required)>'
        ...
```

While we were able to direct them to the proper fix (add listen to
their app's Gemfile), the error message does not really point them in
that direction on its own.

This commit improves the error message by using Kernel#gem to indicate
that the gem should be in the user's bundle. This is the same approach
used for other not-specified dependencies in Rails, such as the various
database drivers in their Active Record adapters.

New error message:
```shell
$ bin/rails r "ActiveSupport::EventedFileUpdateChecker"
/home/hartley/.cache/asdf/installs/ruby/3.2.0/lib/ruby/site_ruby/3.2.0/bundler/rubygems_integration.rb:276:in `block (2 levels) in replace_gem': listen is not part of the bundle. Add it to your Gemfile. (Gem::LoadError)
        from /home/hartley/src/github.com/skipkayhil/rails/activesupport/lib/active_support/evented_file_update_checker.rb:3:in `<top (required)>'
        from /home/hartley/.cache/asdf/installs/ruby/3.2.0/lib/ruby/gems/3.2.0/gems/zeitwerk-2.6.6/lib/zeitwerk/kernel.rb:38:in `require'
        from /home/hartley/.cache/asdf/installs/ruby/3.2.0/lib/ruby/gems/3.2.0/gems/zeitwerk-2.6.6/lib/zeitwerk/kernel.rb:38:in `require'
	...
```

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -1,9 +1,11 @@
 # frozen_string_literal: true
 
+gem "listen"
+require "listen"
+
 require "set"
 require "pathname"
 require "concurrent/atomic/atomic_boolean"
-require "listen"
 require "active_support/fork_tracker"
 
 module ActiveSupport