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

[J.P.]

I've added these changes as

  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=c68dc778

Although I've done so with zero discussion, as usual, others can perhaps
take some comfort in knowing that this semi-major overhaul only reaches
as far back as the latest release (but not into it). Thanks.