Add 'treesit-language-at-point-default' (bug#77256).

Add 'treesit-language-at-point-default' (bug#77256).

* lisp/treesit.el (treesit-language-at-point-function):
Change the default value from nil to 'treesit-language-at-point-default'.
(treesit-language-at): Funcall 'treesit-language-at-point-function'
unconditionally.
(treesit-language-at-point-default): New function with body from
'treesit-language-at'.
(treesit-node-at): Simplify by replacing duplicate code
with the call to 'treesit-parsers-at'.

(cherry picked from commit 77bf9d33ee42d90e86d9cc5d3a67356b19ca2f6b)