[ELPA] New package: ERC

[Amin Bandali]

Hello,

It's about time we added ERC to GNU ELPA. :)

The first attached patch against elpa.git does that.

The second patch allows for a nonexistent erc-loaddefs, since that
file would not generated for the GNU ELPA package, and instead an
erc-autoloads.el would be generated and (IIRC) automatically loaded.

I believe the above two are enough to get ERC onto GNU ELPA, with a
minimum required Emacs version of 27.

I would like to lower that lower bound by a few Emacs major versions
so that users on older Emacsen could install the package as well.
It seems that we could bring this lower bound down to Emacs 25 with
relatively small amount of effort, by:

1. Making the iso8601 dependency optional; or even better, adding it
   to GNU ELPA and adapting it to only use the new functions like
   `string-replace' on newer Emacsen.

   What do you think about this, Lars?

2. Adding browse-url to GNU ELPA, since erc-button uses its
   `browse-url-button-regexp', which was only added in Emacs 27.
   We could probably adjust erc-button to fall back onto Gnus's
   `gnus-button-url-regexp' on older Emacsen, but I think I'd
   rather not do that.

3. Not using `with-suppressed-warnings' (added in Emacs 27) on older
   Emacsen and perhaps fall back on `with-no-warnings' for the single
   use of that macro instead.

It might be possible to further lower the minimum required version of
down to the 24 series, but that would require doing something about
the uses of `define-inline' from inline.el, which was added in
Emacs 25.  Not sure if it would make sense or be possible to get
inline added to GNU ELPA for Emacsen older than 25.  Stefan?

Thanks,
amin

0001-elpa-packages-erc-Add-package.patch
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0001-lisp-erc-erc.el-Don-t-error-if-erc-loaddefs-does-not.patch
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