Add erc-timestamp property to invisible messages

Add erc-timestamp property to invisible messages

* lisp/erc/erc-fill.el (erc-fill--wrap-beginning-of-line): Pretend
nicks with an empty string as a `display' prop are `invisible-p' and
break out of hidden "merged" nicks after moving.  (Bug#60936.)
* lisp/erc/erc-match.el (erc-hide-fools): Add comment.
* lisp/erc/erc-stamp.el (erc-add-timestamp): Always add
`erc-timestamp' and `cursor-sensor-functions' properties but respect
tradition and don't actually stamp any invisible messages.