Improve ERC's handling of multiline prompt input

Improve ERC's handling of multiline prompt input

* lisp/erc/erc.el (erc--pre-send-split-functions): Add new internal
hook allowing members to revise individual lines before sending.  This
was created with an eye toward possibly exporting it publicly as a
customizable option.
(erc-last-input-time): Tweak meaning of variable to match likely
original intent, which is that it's only updated on successful calls
to `erc-send-current-line'.
(erc--discard-trailing-multiline-nulls): Conditionally truncate list
of lines to be sent, skipping trailing blanks.  This constitutes a
behavioral change.  But, considering the nature of the bug being
fixed, it is thought to be justified.
(erc--input-split): Add new internal struct containing split input
lines and flag for command detection.
(erc--input-line-delim-regexp): Add regex var for splitting multiline
prompt input.
(erc--blank-in-multiline-p): Add helper for detecting blank lines.
(erc--check-prompt-input-for-multiline-blanks,
erc--check-prompt-input-for-point-in-bounds,
erc--check-prompt-input-for-running-process): New functions to
encapsulate logic for various pre-flight idiot checks.
(erc--check-prompt-input-functions): Add new hook for validating
prompt input prior to clearing it, internal for now.
(erc-send-current-line): Pre-screen for blank lines and bail out if
necessary.
(erc-send-input): Add optional param to skip checking for blank lines.
Call hook `erc--pre-send-split-functions'.

* test/lisp/erc/erc-tests.el (erc-ring-previous-command): Use new test
helper.
(erc--input-line-delim-regexp,
erc--blank-in-multiline-input-p): Add tests.
(erc-tests--send-prep, erc-tests--set-fake-server-process,
erc-tests--with-process-input-spy): Add test helpers.
(erc--check-prompt-input-functions, erc-send-current-line,
erc-send-whitespace-lines): Add tests.
(Bug#54536)