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

[Juri Linkov]

> It would be nice if the fallback mode is used when treesitter is not
> usable.  Say for example, for the following file variable in a C file:
>
> /* -*- mode: c; mode: c-ts; -*- */
>
> If c-ts-mode is not usable (regardless of whether the mode is
> available), activate c-mode instead.
>
> This bug is regarding the automatically loaded mode when a file with some
> mode is set, and not regarding the user explicitly setting the mode.
>
> Supporting this can help me use multiple mode file variables regardless
> of the GNU Emacs version used by co-developers.

'major-mode-remap-alist' should be able to handle this requirement
since its design is similar to MIME.TYPES and MAILCAP.  But actually
MAILCAP supports an additional predicate TEST that is a command
executed to determine whether or not the mailcap line applies.

So to completely cover all needs, 'major-mode-remap-alist' should
support a predicate as well.  For example,

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

or a more complex predicate:

(setq major-mode-remap-alist '((c-mode (lambda ()
                                         (and (treesit-available-p)
                                              (treesit-ready-p 'c t)))
                                c-ts-mode)))