Port the `ext` Cog

[shtlrs]

Both Python and Sir-Lancebot have the ext command that allows to load/unload/list extensions.

The code is currently duplicated, and seems fit to be ported to bot-core as all bots could use that feature to have access to the same information about the different extensions' status.

[onerandomusername]

it should also be added that sir-robin doesn't even have this command at the moment, and this would give it that ability.