Fix date-stamp regression in erc-fill-wrap

Fix date-stamp regression in erc-fill-wrap

* lisp/erc/erc-fill.el (erc-fill-wrap, erc-fill-wrap-enable)
(erc-fill-wrap-disable): Add and remove `erc-stamp--insert-date-hook'
member.
(erc-fill--wrap-continued-predicate): Add function-valued variable for
modules to influence `erc-fill--wrap-continued-message-p', which was
originally introduced as part of bug#60936.
(erc-fill--wrap-rejigger-last-message): Move toward beginning of file.
(erc-fill--wrap-unmerge-on-date-stamp): New function.
(erc-fill-wrap): Use `erc-fill--wrap-continued-predicate'.  Restore
recently deleted hunk that reset the wrap marker upon seeing a date
stamp.
* test/lisp/erc/erc-scenarios-fill-wrap.el: New file.
* test/lisp/erc/resources/fill/wrap/merge-datestamp.eld: New file.