bug#60623: 30.0.50; Add forward-sentence with tree sitter support

[Stefan Monnier]

> Exactly.  IMO, if the variable can have a
> nonfunction value, especially nil, then

Bad idea.  That precludes changing the value of the variable with
`add-function`, whereas `add-function` is often the best way for
a major/minor mode to change that variable (actually, the "only" way to
do it with some hope that it will interact correctly with other modes
that may change it as well).

> there's no need (nothing gained, and even possible
> confusion/misunderstanding added) by adding "-default-" to the name.

Instead we should strive to make the name of the default function simply
irrelevant.  If all goes well, nobody should ever need to explicitly
call "the default value" of that variable.  Instead the function they
added via `add-function` will receive the "previous" value as argument.

> On the other hand, if the value must always be a function, then having
> "-default-" in the name makes sense.

Agreed.  It's also helpful when you look at the var's value, it lets you
know that it hasn't been modified.


        Stefan