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

[Stefan Monnier]

>> (setq major-mode-remap-alist '((c-mode #'treesit-available-p c-ts-mode)))

The current syntax is rather

    (setq major-mode-remap-alist '((c-mode . treesit-available-p c-ts-mode)))

> I think the idea is that this should happen by default, not as a
> (not-so-trivial) customization by the users.

Agreed.  `major-mode-remap-alist` is a custom var and I think we should
try and keep its default value as close to nil as possible.


        Stefan