Optionally add spacing between ERC messages

Optionally add spacing between ERC messages

* etc/ERC-NEWS: Mention option `erc-fill-line-spacing'.
* lisp/erc/erc-fill.el (erc-fill-line-spacing,
erc-fill-spaced-commands): Add options to allow for extra spacing
between messages.
(erc-fill--function): Internal var allowing modules to override user
option `erc-fill-function'.
(erc-fill): Add extra line-spacing on certain types of messages.
Prefer `erc-fill--function', when set, over `erc-fill-function'.
(erc-fill--make-module-dependency-msg,
erc-fill--wrap-ensure-dependencies): Rename former to latter and make
more useful.
(erc-fill-wrap-mode, erc-fill-wrap-enable, erc-fill-wrap-disable):
Refactor.
(erc-fill--wrap-fix): Remove unused function.
(erc-fill-wrap-nudge): Remove reference to nonexistent function in doc
string.
* test/lisp/erc/erc-fill-tests.el: (erc-fill-tests--graphic-dir): New
variable.
(erc-fill-tests--compare): Look in `erc-fill-tests--graphic-dir' for
graphical snapshots ignored by Git.
(erc-fill-line-spacing): New test.
* test/lisp/erc/resources/fill/snapshots/spacing-01-mono.eld: New
file.  (Bug#60936)