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

[Stefan Monnier]

>> The current syntax is rather
>>
>>     (setq major-mode-remap-alist '((c-mode . treesit-available-p c-ts-mode)))
>
> Is it possible at least to make this forward-compatible to support
> such a predicate in future?  To enable ts-modes only in some buffers
> currently requires first to enable non-ts-mode, then conditionally ts-mode:

We can use:

    (setq (alist-get 'c-mode major-mode-remap-alist)
          (lambda () (if (treesit-available-p) (c-ts-mode) (c-mode))))


-- Stefan