New hook 'outline-after-change-functions' (bug#77256).

New hook 'outline-after-change-functions' (bug#77256).

* lisp/outline.el (outline-after-change-functions): New variable.
(outline--fix-buttons-after-change):
Run hook 'outline-after-change-functions'.

* lisp/treesit.el (treesit--after-change): Improve docstring.
(treesit-major-mode-setup): Simplify to just add
'treesit--after-change' to 'outline-after-change-functions' hook.

* lisp/progmodes/rust-ts-mode.el (rust-ts-mode): Add explicit
'treesit-outline-predicate' that is like
'treesit-simple-imenu-settings', but also adds "trait_item".

(cherry picked from commit 7574eb82c5cd150a0706a81e9f8555962a1e2167)