Reuse command-indicator code for script lines in ERC

Reuse command-indicator code for script lines in ERC

* lisp/erc/erc-goodies.el (erc-load-irc-script-lines): Move here from
main file and rework to always use `command-indicator' instead of only
partially, when available.  Also use internal "send-action" and
"send-message" interfaces to defer command-handler output until
command lines have been inserted.
* lisp/erc/erc.el (erc-process-input-line): Redo doc string.
(erc-process-script-line): Fold exceptionally overlong line.
(erc-load-irc-script-lines): Move to erc-goodies.el.  (Bug#67032)

(cherry picked from commit a46789b56af05e4cd31ab90495c9f2a4492a9b19)