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

[Mohammed Sadiq]

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.


In GNU Emacs 29.0.60 (build 7, x86_64-pc-linux-gnu, GTK+ Version
 3.24.35, cairo version 1.16.0) of 2022-12-18 built on purism
Repository revision: 5b2e6d04ce271cca2de944f8f5c07c005da33e37
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 
11.0.12101004
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure --prefix=/usr'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  value of $LANG: en_IN.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: C

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
warnings icons c-ts-mode treesit cl-seq cc-mode cc-fonts cc-guess
cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
cl-loaddefs cl-lib rmc iso-transl tooltip cconv eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
emacs)

Memory information:
((conses 16 59479 7190)
 (symbols 48 7146 0)
 (strings 32 20640 2145)
 (string-bytes 1 740726)
 (vectors 16 12858)
 (vector-slots 8 184206 12553)
 (floats 8 29 94)
 (intervals 56 239 0)
 (buffers 984 13))