bug#60559: 29.0.60; "Cannot activate tree-sitter" spam

[Theodor Thornhill]

> So here's a suggestion for such a solution: we make all the
> *-ts-mode's optional.  That is, we don't add any of them to
> auto-mode-alist unless the file *-ts-mode.el is loaded, and we
> document them all in NEWS and the user manual as optional.  users who
> want them will have to manually activate them.  This way, the original
> use case that started this bug report is automatically solved, and the
> other use case, where the user intends to activate one of these modes,
> is also served by showing the warning, which in that case is perfectly
> justified: the user asked for something that we cannot do, so we warn
> him/her.
>
> This is a retreat of sorts, but I think it strikes a better balance
> wrt user expectations, assuming not everyone will build with
> tree-sitter.
>
> Any comments or objections?  If not, I will do this in a couple of
> days on the emacs-29 branch.

Sounds good to me :-)