Expand the scheduling API to allow synchronous scheduling

[BomBardyGamer]

I am unsure if this is a good idea or not, but the idea is to allow plugins to schedule things synchronously. Though I don't have any use cases in mind as I write this, and I have many good reasons against this, it may still be worth discussing, which is why I'm writing this issue.

Pros:

• Allows plugins to do some things in sync with the server

Cons:

• Exposes another implementation detail (that the server has ticking logic and a main thread)
• The API is meant to be designed in a way that makes users worry less about concurrency issues, since those are a server-side issue, and focus more on the actual thing they want to do.
• Some coming from Bukkit or Sponge may not understand that most things wouldn't need to be synchronous.
• Increases the complexity of the scheduling API
• There are already events that fire on tick start and end, allowing plugins to do actions in sync with the server.