New function treesit-parsers-at for treesit-language-at (bug#77256).

New function treesit-parsers-at for treesit-language-at (bug#77256).

* doc/lispref/parsing.texi (Multiple Languages): The variable
'treesit-language-at-point-function' is now optional for
multi-language major modes.  Add description of 'treesit-parsers-at'.

* lisp/treesit.el (treesit-language-at-point-function):
Change the the docstring to remove the dissuasion against
deriving the language from parser ranges.
(treesit-language-at): Use the first parser from
'treesit-parsers-at' as the default return value when
'treesit-language-at-point-function' is nil.  Adapt the docstring.
(treesit-node-at): Use 'treesit-parsers-at'.
(treesit-parsers-at): New function.
(treesit-local-parsers-at): Use 'treesit-parsers-at'
with the most part of the body moved to it.
(treesit-local-parsers-on): Replace the overlay property
'treesit-parser' with 'treesit-parser-local-p' in the docstring.
(treesit-up-list, treesit-simple-imenu, treesit-outline-level):
Use 'treesit-parsers-at'.

* lisp/progmodes/c-ts-mode.el (c-ts-mode): Don't set
'treesit-language-at-point-function'.

* lisp/progmodes/elixir-ts-mode.el (elixir-ts--treesit-language-at-point):
Remove.
(elixir-ts-mode): Don't set 'treesit-language-at-point-function'.

* lisp/progmodes/js.el (js--treesit-language-at-point): Remove.
(js-ts-mode): Don't set 'treesit-language-at-point-function'.

* lisp/progmodes/php-ts-mode.el (php-ts-mode--html-language-at-point)
(php-ts-mode--language-at-point): Remove.
(php-ts-mode): Don't set 'treesit-language-at-point-function'.

* lisp/textmodes/mhtml-ts-mode.el (mhtml-ts-mode--language-at-point):
Remove.
(mhtml-ts-mode): Don't set 'treesit-language-at-point-function'.
Use 'treesit-language-at' for mode-line lighter.

(cherry picked from commit 5e0daa1ef77d2a5fe5b65b8f0fa6c4eab83a2498)