Make erc-keep-place-indicator aware of erc-truncate

Make erc-keep-place-indicator aware of erc-truncate

* etc/ERC-NEWS: Entry mentioning `erc-keep-place-indicator-truncation'.
* lisp/erc/erc-goodies.el (erc-keep-place-indicator-truncation): New
option.  Something like this should have accompanied the module's
introduction.
(erc-keep-place-indicator-mode, erc-keep-place-indicator-enable)
(erc-keep-place-indicator-disable): Arrange to take necessary measures
to avoid losing the indicator on `erc--clear-function'.  This module was
first introduced by bug#59943.
(erc--keep-place-move-hook): New variable.
(erc--keep-place-indicator-adjust-on-clear): New function.
(erc-keep-place-move): Try to ensure the overlay resides at the
beginning of a message.  Run hook `erc--keep-place-move-hook'.
* test/lisp/erc/erc-scenarios-keep-place-indicator-trunc.el: New file.
* test/lisp/erc/erc-scenarios-keep-place-indicator.el
(erc-scenarios-keep-place-indicator--follow): Fix missing test
description.  (Bug#72736)

(cherry picked from commit 4d7f41716e1485fb57efc6eac9f45f2879c90266)