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: vim/vim#17299

https://github.com/vim/vim/commit/13bea589a25707c8f9e29b2920410bdcccd79bc5

Co-authored-by: Phạm Bình An <phambinhanctb2004@gmail.com>

- 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: vim/vim#17274

https://github.com/vim/vim/commit/3704b5b58ace1163522188e3228996c15e56820e

Co-authored-by: Phạm Bình An <phambinhanctb2004@gmail.com>

vim-tutor-mode provides a mechanism to write and read interactive
tutorials in vim. It's aim is to replace the venerable vimtutor with a
more modern system.

The plugin's development is maintained at https://github.com/fmoralesc
/vim-tutor-mode

Closes #2351.