Support event priorities for events posted via ModLoader

[Technici4n]

To support this we need to make a few changes to ModContainer. The easiest way to handle this going forward is to ask language providers to give direct access to the event bus. I made it public because there are some valid use cases of it. (I think @shedaniel would like this for architectury, for example). The bus is nullable because it is optional, but not java Optional because I don't want to allocate for each call.

Tested in neoforgedev.