Preserve user markers when inserting ERC date stamps

Preserve user markers when inserting ERC date stamps

* lisp/erc/erc-stamp.el
(erc-stamp--insert-date-stamp-as-phony-message): Ensure existing
user markers aren't displaced by date-stamp insertion.
* lisp/erc/erc.el (erc--insert-line-function): New function-valued
variable for overriding `insert'.
(erc-insert-line): Call `erc--insert-line-function', when non-nil, to
insert line specially.
* test/lisp/erc/erc-scenarios-stamp.el
(erc-scenarios-stamp--on-insert-modify): New assertion helper
function.
(erc-scenarios-stamp--date-mode/left-and-right): New test.
(Bug#60936)