Embed elixir in heex as well as elixir->heex->elixir (bug#76788).

Embed elixir in heex as well as elixir->heex->elixir (bug#76788).

* lisp/progmodes/elixir-ts-mode.el (elixir-ts--range-rules):
Rename to a shorter name from 'elixir-ts--treesit-range-rules'.
(elixir-ts--font-lock-feature-list, elixir-ts--thing-settings)
(elixir-ts--range-rules): New variables with default values
extracted from 'elixir-ts-mode'.
(elixir-ts-mode): Use 'elixir-ts--font-lock-feature-list',
'elixir-ts--thing-settings', 'elixir-ts--range-rules'
and 'heex-ts--range-rules'.
Use 'treesit-merge-font-lock-feature-list' to merge
'heex-ts--font-lock-feature-list'.

* lisp/progmodes/heex-ts-mode.el
(heex-ts--font-lock-feature-list, heex-ts--range-rules):
New variables.
(heex-ts-mode): Use 'heex-ts--font-lock-feature-list',
'heex-ts--range-rules'.  Merge 'elixir-ts--font-lock-settings',
'elixir-ts--font-lock-feature-list', 'elixir-ts--thing-settings'
for embedding elixir in heex.  Enable the 'sexp' navigation
by default with 'treesit-cycle-sexp-type'.

* lisp/progmodes/c-ts-mode.el (c-ts-mode): Append
'treesit-range-rules' to possibly already existing list in
'treesit-range-settings'.

* lisp/treesit.el (treesit-language-at-point-default):
Optimize to use 'when-let*'.

(cherry picked from commit d56e37c83c721c5bcffcf434138b27482b7e3fa6)