Require the webrick handler when webrick is available

Followup to #23

Even when users do add webrick to their gemfile, the handler is not available.
This means that they either need to do that themselves, or dependencies need to
be updated.

lib/rackup.rb

@@ -6,3 +6,16 @@
 require_relative 'rackup/handler'
 require_relative 'rackup/server'
 require_relative 'rackup/version'
+
+begin
+  # Although webrick is gone from Ruby since 3.0, it still warns all the way
+  # through to 3.3. Only on 3.4 will requiring it not produce a warning anymore.
+  verbose, $VERBOSE = $VERBOSE, nil
+  require 'webrick'
+  # If the user happens to have webrick in their bundle, make the handler available.
+  require_relative 'rackup/handler/webrick'
+rescue LoadError
+  # ¯\_(ツ)_/¯
+ensure
+  $VERBOSE = verbose
+end