bug#60936: 30.0.50; ERC >5.5: Add erc-fill style based on visual-line-mode

[tzakmagiel]

+1'ing this the issue in Message #166 for visibility. I raised this question 
under the nick "alcor" on #erc yesterday, and I agree that the default behavior 
of `fill-wrap' (i.e. without `scrolltobottom') might be confusing/unexpected 
for new `fill-wrap' users (such as myself, in this case).

For the record, the behavior without the scrolltobottom module could be 
described as "messages tend to drift upward on screen, gradually increasing the 
whitespace between prompt and bottom of window" (This description courtesy of 
corwin on #erc).

>I'm thinking it might make sense to have `fill-wrap' formally depend on 
>`scrolltobottom', even though there's no technical reason to do so.

+1 on that too. The behavior with `scrolltobottom' makes more sense (as a 
default) and is more in line with other IRC clients, where the message prompt 
is kept at the bottom of the window.