Inhibit slash commands in erc--input-review-functions

Inhibit slash commands in erc--input-review-functions

* lisp/erc/erc.el (erc--input-review-functions): Add new review
function `erc--inhibit-slash-cmd-insertion'.
(erc--check-prompt-input-functions): Move
`erc--check-prompt-input-for-multiline-command' above
`erc--check-prompt-input-for-multiline-blanks'.
(erc--inhibit-slash-cmd-insertion): New "review" function to suppress
insertion of prompt input for slash commands.  Doesn't affect "meta"
slash commands like /SAY.
(erc--send-input-lines): Don't bother checking whether message is a
command.  Instead, trust verdict handed down by message-prep and
review functions.  This opens the door to optional insertion for
debugging purposes or when echoing command lines in a shell-like
fashion.
* test/lisp/erc/erc-tests.el (erc-send-whitespace-lines): clean up
portion dealing with trimming slash commands.  (Bug#66073)