Add replacement-text field to erc-input struct

Add replacement-text field to erc-input struct

* etc/ERC-NEWS: Promote `refoldp' slot from simulated to real.
Mention new `substxt' slot of `erc-input' struct.
* lisp/erc/erc-common.el (erc-input): Add `substxt' and `refoldp'
slots.
(erc--input-split): Move `refoldp' to "superclass".
* lisp/erc/erc-goodies.el (erc--command-indicator-permit-insertion):
Use `substxt' field instead of overloading `insertp'.
(erc--command-indicator-display): Accept extra lines for
compatibility.
* lisp/erc/erc.el (erc-pre-send-functions): Revise doc.
(erc--input-ensure-hook-context, erc-input-refoldp): Remove unused
functions, originally meant to be new in ERC 5.6.
(erc--run-send-hooks): Copy data from additional fields of `erc-input'
object to `erc--input-split' workspace object.
(erc--send-input-lines): Handle `substxt' field of `erc-input' object
when it's non-nil.  (Bug#68265)