Make nested input handling more robust in ERC

Make nested input handling more robust in ERC

* lisp/erc/erc.el (erc--send-action-function): New function-valued
variable for locally advising `erc-send-action' so that built-in
modules can elect to handle insertion and sending themselves.
(erc-send-action): Defer to `erc--send-action-function'.
(erc--send-action-perform-ctcp): Isolate the message-sending business
for CTCP ACTIONs that used to reside in `erc-send-action'.
(erc--send-action-display): Isolate the message-insertion business
formerly residing in `erc-send-action' for more granular use.  Fix a
minor bug involving inserted representations of CTCP ACTIONs not
having `erc-my-nick-face' applied to the speaker.
(erc--send-action): Perform the same displaying and sending of CTCP
ACTION messages formerly handled by `erc-send-action', but display
messages before sending them.
(erc--current-line-input-split): New variable bound to the post-review
`erc--input-split' object for the extent of display processing.  This
mainly benefits slash-command handlers and the utility functions they
employ, such as `erc-send-message'.
(erc-cmd-SAY): Defer to `erc--send-message'.
(erc--send-message-nested-function): New function-valued variable
supporting an internal interface for influencing how
`erc-send-message' inserts and sends prompt input.  Some handlers for
slash commands, like /SV, use `erc-send-message' to perform their own
insertion and sending, which is normally the domain of
`erc-send-current-line'.  When this happens, modules can't easily
leverage the normal hook-based API to do things like suppress
insertion but allow sending or vice-versa.  This variable provides an
internal seam for modules to exert such influence.
(erc-send-message): Behave specially when called by the default
interactive client via `erc-send-current-line' and friends.
(erc--send-message-external): New function to house the former body of
`erc-send-message', for third-party code needing to apply the
traditional behavior.
(erc--send-message-nested): New function for turning arbitrary text,
such as replacement prompt input, into outgoing message text by doing
things like ensuring "send" hooks run and invariants for prompt
markers are preserved.
(erc--make-input-split): New helper function for creating a standard
`erc--input-split' object from a string.  This is arguably less
confusing than adding another constructor to the struct definition.
(erc-send-current-line): Bind `erc--current-line-input-split' when
dispatching prompt-input handlers.  Use helper `erc--make-input-split'
to initialize working `erc--input-split' state object.
(erc--run-send-hooks): Honor existing `refoldp' slot from
`erc--input-split' object.
(erc--send-input-lines): Convert to generic function to allow modules
control over fundamental insertion and sending operations, which is
necessary for next-generation features, like multiline messages.
(erc-modes): Don't output non-modules.  That is, only list actual
modules created via `define-erc-module', and `quote' members of the
resulting list.
* test/lisp/erc/erc-scenarios-base-send-message.el: New test file.
* test/lisp/erc/resources/base/send-message/noncommands.eld: New data
file.  (Bug#67031)