bug#57955: 29.0.50; Allow session-local ERC modules

[J.P.]

I've added something similar to the proposed change as

  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=d46c016f

If you'll recall, I initially left this bug open as a reminder that we
still lack a solution to the glaring problem of configuration scoping.
To summarize, this concerns the pressing need to allow users to specify
different values for new (and hopefully existing) global options based
on context (like network, channel, speaker, etc.). One idea bandied
about has been adapting connection-local variables to be more abstract
and eventually integrating them with Customize and `use-package'. As
thing stand, though, each local module must itself decide whether it's
session-local, buffer-local, or both. And it must contend with stashing
and restoring its configured state on its own.

I'll likely close this bug after opening another to address these
broader configuration concerns. Thanks.