runtime(tutor): improve tutor.vim plugin and filetype plugin

- Set g:tutor_debug on startup if it doesn't exist so that users can get
  cmdline completion when interactively setting it.
- set b:undo_ftplugin in filetype plugin
- set default runtime file headers

closes: #17274

Signed-off-by: Phạm Bình An <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/plugin/tutor.vim

@@ -1,6 +1,17 @@
+" Tutor:	New Style Tutor Plugin :h vim-tutor-mode
+" Maintainer:	This runtime file is looking for a new maintainer.
+" Contributors:	Phạm Bình An <phambinhanctb2004@gmail.com>
+" Original Author: Felipe Morales <hel.sheep@gmail.com>
+" Date: 2025 May 10
+
 if exists('g:loaded_tutor_mode_plugin') || &compatible
     finish
 endif
 let g:loaded_tutor_mode_plugin = 1
 
+" Define this variable so that users get cmdline completion.
+if !exists('g:tutor_debug')
+  let g:tutor_debug = 0
+endif
+
 command! -nargs=? -complete=custom,tutor#TutorCmdComplete Tutor call tutor#TutorCmd(<q-args>)