[luatex] adjusting `hyphenate` callback for single function

[Javier Bezos]

> * I couldn't find information in the manual about the order of hooks.
>    It seems that I have to save the original hook by myself and call it
>    before/after, right?
> 
> * Is there already some code available that I could use as a starting
>    point?

They are very old and the syntax has changed in some lua
commands (and a callback is no longer available), but I
found the following articles very useful when I was
learning how the callbacks work:

https://tug.org/tugboat/tb31-3/tb99isambert.pdf

https://tug.org/TUGboat/tb32-1/tb100isambert.pdf

Javier