v2. Revive `erc-command-indicator' functionality as new module.

"J.P." <jp@neverwas.me> writes:

> In tackling this, I've opted to sidestep a few related concerns that
> will have to be dealt with eventually. The first has to do with
> discovering and possibly reintegrating whatever role the `noncommands'
> module is designed to play in processing specialty slash commands, like
> those reliant on `erc-send-message'. These days (reaching as far back as
> Emacs 26.1), the module appears to effectively be a no-op, at least with
> the default client configuration. Until we know more, I'm inclined to
> let it rot away in erc-goodies, since its code only runs on interactive
> input anyway.

Actually, I've gone ahead and done some cursory spelunking in hopes of
shedding some light on this `noncommands' quandary. First off, there's
no relevant mention of `erc-display-command' or `erc-command-indicator'
in the old erc-discuss archives. But it's my current understanding that
the module seems to have been effectively sidelined when the lone call
site for `erc-display-command' was deep sixed in

  d1036d288de backport: erc bugfixes

(This was back in November of 2013, under 5.3.x and what became Emacs
24.5.) Strangely, that commit did not include any change log or ERC-NEWS
mentions, which is unfortunate because it's wide-ranging and largely
nontrivial. For example, it left behind related faces and options, like
`erc-command-indicator', thereupon effectively neutering them along with
`noncommands'. More recently, `erc-display-command' was commented out in

  0c599ee2e2c * lisp/erc/erc.el: Use `run-hook-with-args` for
              `erc-pre-send-functions`

and later removed by

  a63ed6f78a6 Remove duplicate ERC prompt on reconnect

but the damage had long since been done. My point in drudging this up is
merely to highlight where the behavior concerned diverged and that we'll
likely never know why. I really have no interest in relitigating ancient
motivations and methods. (It's quite possible that discussions did take
place elsewhere, like on Freenode or the old sourceforge list.)

Anyway, since both the interface (options, fonts) and infrastructure are
still in place, I think it behooves us to revive this functionality, not
least as a signal to others that ERC picks up after itself (eventually).
I've thus added some changes to this effect (see last patch).

Screenshot:

https://debbugs.gnu.org/cgi/bugreport.cgi?msg=6;filename=erc-command-indicator.png;att=1;bug=67032