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/ftplugin/tutor.vim

@@ -6,21 +6,13 @@
 " Original Author: Felipe Morales <hel.sheep@gmail.com>
 " Last Change:
 " 2025 May 10 set b:undo_ftplugin
+" 2025 May 12 update b:undo_ftplugin
 
 " Base: {{{1
 call tutor#SetupVim()
 
 " Buffer Settings: {{{1
 setlocal noreadonly
-if !exists('g:tutor_debug') || g:tutor_debug == 0
-    setlocal buftype=nofile
-    setlocal concealcursor+=inv
-    setlocal conceallevel=2
-else
-    setlocal buftype=
-    setlocal concealcursor&
-    setlocal conceallevel=0
-endif
 setlocal noundofile
 
 setlocal keywordprg=:help
@@ -46,14 +38,7 @@ call tutor#SetNormalMappings()
 sign define tutorok text=✓ texthl=tutorOK
 sign define tutorbad text=✗ texthl=tutorX
 
-if !exists('g:tutor_debug') || g:tutor_debug == 0
-    call tutor#ApplyMarks()
-    autocmd! TextChanged,TextChangedI <buffer> call tutor#ApplyMarksOnChanged()
-endif
-
-let b:undo_ftplugin = 'unlet! g:tutor_debug |'
-let b:undo_ftplugin ..= 'setl concealcursor< conceallevel< |'
-let b:undo_ftplugin ..= 'setl foldmethod< foldexpr< foldlevel< |'
-let b:undo_ftplugin ..= 'setl buftype< undofile< keywordprg< iskeyword< |'
+let b:undo_ftplugin = "setl foldmethod< foldexpr< foldlevel< undofile< keywordprg< iskeyword< |"
+    \ . "call tutor#EnableInteractive(v:false) |"
 
 " vim: fdm=marker