bug#57548: Add new function `seq-positions'

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

>> I changed the text in the manual, NEWS and docstring to talk about
>> "index" instead of "position".  I kept the word "positions" in the
>> function name because there is already a `seq-position` function and the
>> 2 are so similar that I think they deserve a similar name. What do you
>> think?
>
> LGTM, thanks.

Me too.