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/autoload/tutor.vim

@@ -210,6 +210,7 @@ function! tutor#TutorCmd(tutor_name)
 
     call tutor#SetupVim()
     exe "edit ".l:to_open
+    call tutor#EnableInteractive(v:true)
 endfunction
 
 function! tutor#TutorCmdComplete(lead,line,pos)
@@ -217,3 +218,24 @@ function! tutor#TutorCmdComplete(lead,line,pos)
     let l:names = uniq(sort(map(l:tutors, 'fnamemodify(v:val, ":t:r")'), 's:Sort'))
     return join(l:names, "\n")
 endfunction
+
+" Enables/disables interactive mode.
+function! tutor#EnableInteractive(enable)
+    let enable = a:enable
+    if enable
+        setlocal buftype=nofile
+        setlocal concealcursor+=inv
+        setlocal conceallevel=2
+        call tutor#ApplyMarks()
+        augroup tutor_interactive
+            autocmd! TextChanged,TextChangedI <buffer> call tutor#ApplyMarksOnChanged()
+        augroup END
+    else
+        setlocal buftype<
+        setlocal concealcursor<
+        setlocal conceallevel<
+        if exists('#tutor_interactive')
+            autocmd! tutor_interactive * <buffer>
+        endif
+    endif
+endfunction