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

[Juri Linkov]

>>> >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.
>>
>>On the other hand, if the value must always
>>be a function, then having "-default-" in
>>the name makes sense.
>
> So is this to be considered an improvement to forward-sexp too, then?

Sorry, I can't find where a lambda is set to forward-sexp-function.
I only see this:

  (defvar forward-sexp-function nil

But if it will be set to a function later, it would be nice
to define a default function as well.