Add line-wise movement commands for erc-fill-wrap

Add line-wise movement commands for erc-fill-wrap

* lisp/erc/erc-fill.el (erc-fill-line-spacing): Revise doc string.
(erc-fill--wrap-escape-hidden-speaker): New helper function to move
point to beginning of visible text.
(erc-fill--wrap-beginning-of-line): Factor out adjustment for hidden
speakers.
(erc-fill--wrap-previous-line, erc-fill--wrap-next-line): Add commands
for moving to previous and next line in a manner consistent with the
value of `erc-fill--wrap-visual-keys'.
(erc-fill-wrap-mode-map): Add remap bindings for `next-line' and
`previous-line'.
(erc-fill-wrap-mode): Revise doc string.
(erc-fill-wrap-nudge): Fix vertical anchoring so that point's line
remains steadier throughout the adjustment.  (Bug#60936)