plugin: keep old-school plugin devs happy w/plural command decorators

[dgw]

Description

Enough people expressed dismay that our upcoming migration to sopel.plugin (from sopel.module) would get rid of the plural commands decorator family. So we can have both, and plugin devs may choose depending on how they decorate stuff.

@plugin.command('justone')
def one_name(bot, trigger):
    pass

@plugin.commands('multiple', 'names')
def two_names(bot, trigger):
    pass

I gave the aliases very short docstrings that just link back to the main entry, so Sphinx wouldn't duplicate the whole shebang.
Locally built preview:

@Exirel, is adding tests for the aliases worthwhile? I don't feel like bothering. 😝

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[Exirel]

You need to update sopel.module's imports as well.

[dgw]

@Exirel Wait, so now you're in favor of touching deprecated stuffs? 😼

[Exirel]

@dgw your PR is about un-deprecating commands, tho.

[dgw]

@Exirel plugin.commands wasn't deprecated; it never existed (except briefly in an unreleased state, between #1898 and #1924, for about a month). It's just, after the little row we had over touching deprecated code in #1944, I was surprised that you wanted me to do so here. It makes sense. Importing with as is silly if not needed. Just didn't expect it.