bug#60176: 29.0.60; Fallback file variable mode if treesitter is not usable

[Juri Linkov]

>> (defcustom treesit-enable-modes nil
>>   :type '(repeat
>>           (choice (function-item c-ts-mode)
>>                   (function-item c++-ts-mode)
>>                   (function-item c-or-c++-ts-mode)
>>                   ...
>>              ('c-ts-mode
>>               (when (treesit-ready-p 'c t)
>>                 (add-to-list 'major-mode-remap-alist '(c-mode . c-ts-mode))))
>
> And this bit is completely unacceptable, from where I stand: it
> basically means that the user activated a certain major mode he/she
> wanted to use, but the result could be that an entirely different mode
> was silently activated instead.  What kind of UX is that, and for a
> shining new feature at that??

It could update 'auto-mode-alist' instead of 'major-mode-remap-alist'.
>From the user's point of view this would be more manageable than
what you proposed on emacs-devel with some obscure logic of activating
ts modes when the package is loaded or when the mode is enabled first time.