Measure stamps pixel-wise with erc-fill-wrap

Measure stamps pixel-wise with erc-fill-wrap

* lisp/erc/erc-fill.el (erc-fill-wrap-mode, erc-fill-wrap-enable):
Change "how" for advice applied to `erc-stamp--insert-date-function'
from `:filter-args' to `:after'.
(erc-fill--wrap-stamp-insert-prefixed-date): Change signature to
conform to `:after'-style advice.  Use `buffer-text-pixel-size' when
desired and available for generating display spec.
(erc-fill-wrap): Remove confusing comment.  Don't apply text props to
newlines because there's no obvious use case for inheriting these.
* lisp/erc/erc.el (erc-send-action, erc-display-msg): Add
`erc-command' property (as "PRIVMSG") to inserted message, sans
newline, so that all inserted messages in an ERC buffer contain this
property.
* test/lisp/erc/resources/fill/snapshots/merge-01-start.eld: Update data.
* test/lisp/erc/resources/fill/snapshots/merge-02-right.eld: Update data.
* test/lisp/erc/resources/fill/snapshots/monospace-01-start.eld: Update.
* test/lisp/erc/resources/fill/snapshots/monospace-02-right.eld: Update.
* test/lisp/erc/resources/fill/snapshots/monospace-03-left.eld: Update.
* test/lisp/erc/resources/fill/snapshots/monospace-04-reset.eld: Update.
* test/lisp/erc/resources/fill/snapshots/spacing-01-mono.eld: Update.
(Bug#60936)