Respect existing invisibility props in erc-stamp

Respect existing invisibility props in erc-stamp

* etc/ERC-NEWS: mention `erc-match-toggle-hidden-fools' and new
merging behavior when handling `invisible' text property.
* lisp/erc/erc-match.el (erc-hide-fools): change `invisible' property
to `erc-match' for all messages, not just those with offset bounds.
(erc-match--modify-invisibility-spec): Fix error in doc string.
(erc-match-toggle-hidden-fools): New command.
* lisp/erc/erc-stamp.el (erc-stamp--invisible-property): Add new
internal variable to hold existing `invisible' property merged with
the one registered by this module, the non-namespaced `timestamp'.
(erc-stamp--skip-when-invisible): Add new internal variable, an escape
hatch for pre-ERC-5.6 behavior in which timestamps were not applied at
all to invisible messages.  This led to strange-looking, uneven logs,
and it prevented other modules from offering toggle functionality for
invisibility-spec members registered to them.
(erc-add-timestamp): Merge with existing `invisible' property, when
present, instead of clobbering, but only when escape hatch
`erc-stamp--skip-when-invisible' is nil.
(erc-insert-timestamp-left, erc-format-timestamp): Use possibly merged
`invisible' prop value.  Don't bother with `isearch-open-invisible',
which only affects overlays.
(erc-insert-timestamp-right): Bind `buffer-invisibility-spec' to nil
when figuring `current-column'.  Apply `invisible' text prop to white
space around stamp.
* test/lisp/erc/erc-scenarios-match.el: Require `erc-fill' and
`erc-stamp'.
(erc-scenarios-match--invisible-stamp): Move common setup and core
assertions for some stamp and invisibility-related tests into a
fixture-like helper.
(erc-scenarios-match--stamp-left-fools-invisible): Fix temporarily
disabled test and use fixture.
(erc-scenarios-match--find-eol): New helper.
(erc-scenarios-match--stamp-right-fools-invisible,
erc-scenarios-match--stamp-right-invisible-fill-wrap,
erc-scenarios-match--stamp-both-invisible-fill-static): New tests.
(Bug#64301)