Improve new connections in erc-handle-irc-url

Improve new connections in erc-handle-irc-url

* doc/misc/erc.texi: Add new Integrations section to the info manual
under Advanced Usage.
* etc/ERC-NEWS: Add new section mentioning improved UX when clicking
on irc:// links.

* lisp/erc/erc.el (erc-handle-irc-url): Add optional "scheme"
parameter.  Fix `erc-open' invocation so that the server buffer is
named correctly by deferring to a new customizable opener.  Arrange
for JOINing a channel in a manner similar to ERC's autojoin module.
(erc-url-connect-function): Add new option for creating a new ERC
connection based on info parsed from a URL.
(erc--url-default-connect-function): New function to serve as an
interactive-only fallback when a user hasn't specified a URL connect
function.
* lisp/erc/erc-compat.el (erc-compat--29-browse-url--irc): Add new
compatibility function for `browse-url-irc' and include it in
`browse-url-default-handlers' on Emacs versions below 29.

* test/lisp/erc/erc-tests.el (erc-tests--make-server-buf,
erc-tests--make-client-buf): Add helpers for creating dummy ERC
buffers.
(erc-handle-irc-url): Add test.
* test/lisp/erc/erc-scenarios-misc.el (erc-scenarios-handle-irc-url):
Add new test.
* test/lisp/erc/resources/join/legacy/foonet.eld: Relax
timeout.  (Bug#56514.)