On treesit preference for "consumer" modes

[JD Smith]

The current LANG-mode and LANG-ts-mode separation leaves some ambiguities.  One 
in particular: some modes are “consumers” of other majors modes.  That is, they 
have a different focus, but use LANG modes as subordinate helpers for some 
aspect of their feature set.  Examples: org-mode SRC block highlighting, or a 
shell mode which wants to highlight and indent code written at the shell 
prompt.  In the pre-treesit era, such a “consumer mode” simply reached for, 
e.g., LANG-mode for this, directly making use of all the customizations users 
have applied.

In the new era of LANG-mode and LANG-ts-mode duality, how should the consumer 
mode decide which one to use?

1. Obviously you can check `(treesit-available-p)', but this tells you nothing 
about whether the user prefers and has configured treesit support in their LANG 
buffers.  

2. You can be more specific with `(treesit-ready-p LANG)', but again that only 
says whether treesit with LANG is possible, not whether it is desired or 
configured.

3. You could check `major-mode-remap-alist' for a LANG entry, which does 
indicate affirmative user desire for treesit, but 

  a) this is not required to configure LANG-ts-mode for use in LANG buffers, and
 
  b) it’s not clear this variable or its active usage will persist for the long 
term, in particular if LANG-mode is eventually deprecated in favor of 
LANG-ts-mode.

4. You could force the user to opt-in, with a custom setting like 
`some-consumer-mode-support-treesit', but that requires users to spot this 
option, and will quite reasonably confuse users who have already configured 
treesit support in their LANG buffers.  Why should they have to repeat 
themselves like that?

5. You could combine #3 with checking `auto-mode-alist', but you’d have to 
interpret file extension regexes, which is likely error pone, and leaves out 
dir-local variables, etc.

Is there some long-term durable, central setting or function which consumer 
modes can consult to determine whether to use LANG-mode or LANG-ts-mode?

I guess I’m looking for something like `(treesit-preferred LANG)’ or 
`(default-major-mode LANG)’.