bug#81550: 30.2; add project-specific hooks to unload-feature-special-hooks

> Cc: [email protected]
> Date: Tue, 04 Aug 2026 15:28:33 +0800
> From: Lin Jian via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <[email protected]>
> 
> 
> When supporting `unload-feature' for my project backend, I have to write
> a FEATURE-unload-function to clean `project-find-functions' and
> `project-list-exclude'.
> 
> That FEATURE-unload-function can be omitted if `project-find-functions'
> and `project-list-exclude' are added to `unload-feature-special-hooks'.
> I suggest adding them.

Does it really happen frequently that unloading a feature affects
which files project.el will consider, or is that a rare corner case?
If the latter, perhaps it isn't bad that one needs to write a
FEATURE-unload-function in those cases.