[ESS] fill-paragraph on documentation in ESS/julia

[Lionel Henry]

Hi Tamas,

There has been some work to make fill-paragraph work better within
ROxygen commands and with R code (it can reorganise arguments list of
function and expressions separated by operators).

It's currently activated in a dodgy advice. See

https://github.com/emacs-ess/ESS/blob/08d9d7c05a72c739297bfccd13a0f586e8d6ba87/lisp/ess-roxy.el#L766-L829

I think we should make a key substitution for "M-q" in ess-mode.el.
This would call `ess-fill-paragraph`, which would dispatch to a
`ess-fill-paragraph-function` that could be defined in each language.

This way you'll be able to implement your own modifications for Julia
mode. If you manage to write such a function, please send a pull
request and we'll set up the dispatching mechanism.

Lionel