Optionally prevent sending multiline input in ERC

Optionally prevent sending multiline input in ERC

* lisp/erc/erc.el (erc-inhibit-multiline-input): Add option to cap the
number of lines to be sent before admonishing the user.
(erc-ask-about-multiline-input): Add option to ask instead of warning
user when `erc-inhibit-multiline-input' is reached.
(erc--check-prompt-input-for-excess-lines): Add validator to possibly
warn when too many lines are submitted for transmission.

* test/lisp/erc/erc-tests.el
(erc--check-prompt-input-for-excess-lines): Add test.
(Bug#54536)