Redo line splitting for outgoing messages in ERC

Redo line splitting for outgoing messages in ERC

* lisp/erc/erc-backend.el (erc--reject-unbreakable-lines): New
variable, an escape hatch for somewhat regaining pre-5.6
line-splitting behavior.
(erc--split-line): New utility function that doesn't rely on
column-oriented filling.
* lisp/erc/erc.el (erc--pre-send-split-functions): Append
`erc--split-lines' to value.
(erc--split-lines): New function to re-split current selection of
lines.
(erc-send-input): Hard-code line preparation instead of calling
`erc--pre-send-split-functions', in order to bake in traditional
behavior before move to "pre-splitting".
* test/lisp/erc/erc-scenarios-base-split-line.el: New file.
* test/lisp/erc/erc-tests.el (erc--split-line): New test.
(erc-send-current-line): Don't expect a flood argument when
interpreting a command because it's not passed along to the command's
handler.  This was previously misleading because it assigned undue
significance to something that had no bearing on the fate of a
command.
* test/lisp/erc/resources/base/flood/ascii.eld: New file.
* test/lisp/erc/resources/base/flood/koi8-r.eld: New file.
* test/lisp/erc/resources/base/flood/utf-8.eld: New file.
* test/lisp/erc/resources/erc-d/erc-d.el: Don't decode input.
(Bug#62947)