patch 9.1.1384: still some problem with the new tutors filetype plugin

Problem:  still some problem with the new tutors filetype plugin
Solution: refactor code to enable/disable tutor mode into
          tutor#EnableInteractive() function, include a test
          (Phạm Bình An)

I find it annoying that Tutor's interactive mode is always on (or debug
mode is off) even when I open a tutor file with :edit command.
I think it makes more sense to make this "interactive mode":

- Always on when it is opened with :Tutor command
- Off otherwise

For more references, see `:help` feature, it is a much better than
:Tutor, since I don't have to run `:let g:help_debug = 1` just to be able
to edit and save a help file

Therefore, I remove `g:tutor_debug`

closes: #17299

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

runtime/plugin/tutor.vim

@@ -2,16 +2,11 @@
 " 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
+" Date: 2025 May 12
 
 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>)