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

[Drew Adams]

> >A good practice is to name such a function
> >e.g. forward-sentence-default-function.
> 
> Is this practice used anywhere else? Iirc
> forward-sexp-function doesn't follow that
> practice.

Exactly.  IMO, if the variable can have a
nonfunction value, especially nil, then
there's no need (nothing gained, and even
possible confusion/misunderstanding added)
by adding "-default-" to the name.

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

Following this guideline helps users know
(guess) what the variable's value can be
even without consulting its doc.