Respect user markers in erc-insert-timestamp-left

Respect user markers in erc-insert-timestamp-left

* lisp/erc/erc-stamp.el (erc-insert-timestamp-left): Convert to normal
function, a mere wrapper that defers to existing generic variants, in
order to dissuade users from adding their own methods, which could
complicate troubleshooting, etc.
(erc--insert-timestamp-left): Rename both methods using internal
double-hyphen convention.  In `erc-stamp--display-margin-mode'
implementation, don't displace third-party markers.
* test/lisp/erc/erc-scenarios-stamp.el: New file.  (Bug#60936)