[core] abstract sending a direct message for all Discord APIs

This involves work in all of the modules of disparse, namely:

* disparse-core
* disparse-jda
* disparse-d4j
* disparse-smalld

The goal of this feature is to abstract sending a direct message to a user. In disparse-core, there should be a method added to AbstractDispatcher which is an abstract method left to be implemented by each of jda, d4j, and smalld.

Then in each Dispatcher class for each module, there will be an implementation specific to the underlying library. Also, don't forget about the Dispatcher that is in `disparse-core/src/test/java/disparse/discord/TestDispatcher.java`

This feature can be tested manually for now using the `disparse-testing` module.