Add variant for erc-match invisibility spec

Add variant for erc-match invisibility spec

* lisp/erc/erc-match.el (erc-match-mode, erc-match-enable,
erc-match-disable): Arrange for possibly adding or removing
`erc-match' from `buffer-invisibility-spec'.
(erc-match--hide-fools-offset-bounds): Add new variable to serve as
switch for activating invisibility on a modified interval that's
offset toward `point-min' by one character.
(erc-hide-fools): Optionally offset start and end of invisible region
by minus one.
(erc-match--modify-invisibility-spec): New housekeeping function to
set up and tear down offset spec.  (Bug#60936.)