Remove dependence on static singleton Instance of SisbaseBot

[wffirilat]

In particular, the areas where SisbaseBot.Instance are currently used are the SMC (being the legacy code that it is), and to access Sisbase infrastructure from commands (which belong to D#+.CommandsNext, and currently have no way to access the bot).

• The conversion of SMC to a property on each bot instance should be fairly trivial
• Accessing the "active" sisbase instance from within a command may be harder
  • currently, we are investigating CommandsNext dependency injection as a way to deliver the instance to the command, but it's possible that if D#+ doesn't allow a way to avoid boilerplate while doing this, we will reject that solution and have to find something else. Boilerplate is unacceptable.

[wffirilat]

After investigation, there is no way to avoid boilerplate being required in every command module class when using dependency injection through CommandsNext. As such, that solution, though it would be the right thing to do if it worked, has been rejected.
Currently, no acceptable alternatives are known. Further investigation is required.

[alikindsys]

Boilerplate issues using default cnext dependency injection fixed on DSharpPlus/DSharpPlus#639.
That solution can now be used.