Tidy up ERC's internal text-property API

Tidy up ERC's internal text-property API

* lisp/erc/erc-fill.el (erc-fill--spaced-commands): Remove unused
internal variable originally intended for ERC 5.6.
(erc-fill): Check for `erc-msg' being `msg', which carries the same
meaning as `erc-cmd' being `PRIVMSG' or `NOTICE', except that inserted
outgoing messages now no longer normally have an `erc-cmd' property.
(erc-fill-wrap-mode, erc-fill-wrap-disable): Kill
`erc-fill--wrap-last-msg'.
(erc-fill--wrap-max-lull): Convert from buffer-local to normal
variable.
(erc-fill--wrap-continued-message-p): Rework slightly to guard against
resetting the "last speaker" marker when the `erc-ephemeral' text
property is present.  This tells insert- and send-related hook members
to pretend the current message doesn't exist when performing stateful
operations.  That is, modules should expect the message being inserted
to possibly disappear or be replaced.  Also, look for `erc-msg' being
`msg' instead of `erc-cmd' being `PRIVMSG', and fix bug involving only
checking `erc-ctcp' in the current message.
* lisp/erc/erc-stamp.el (erc-add-timestamp): Don't insert timestamps
when the `erc-ephemeral' text property is present.
* lisp/erc/erc.el (erc--msg-props): Add doc string explaining the
purpose of this variable and the various text properties most commonly
present in its value.
(erc--msg-prop-overrides): Add doc string.
(erc-send-action): Don't set `erc-cmd' prop on outgoing CTCP ACTIONs.
(erc-display-message): Reverse overrides to prefer items toward the
front of the alist.
(erc-process-ctcp-query): Include existing overrides from environs.
(erc-send-current-line): Include existing overrides from environs.
(erc-display-msg): Fix doc string and reverse overrides.
* test/lisp/erc/erc-fill-tests.el (erc-fill-tests--insert-privmsg):
Remove stray comment.
(erc-fill-tests--save-p): Set value from environment variable.
(erc-fill-tests--compare): Limit writing snapshots to one test at a
time.
(erc-fill-wrap--merge-action): Fix expected output for non-action
messages that follow action messages.  These were previously merged
but escaped detection.
* test/lisp/erc/resources/fill/snapshots/merge-wrap-01.eld: Update.
(Bug#60936)