[B+C] Improve and simplify sender.sendMessage. Adds BUKKIT-4480

**PR Justification**
At the moment we have two functions that *sorta* do exactly the same thing. I thought, why not replace this with one function that encompasses all the features of the first two, and even adds more.

**PR Rundown**
This PR replaces both of the current `sender.sendMessage(String([]) message)` functions with one, namely `sender.sendMessage(String... messages)`. As the JavaDocs say, I've also added the ability for developers to send new-line messages by simply doing `sender.sendMessage()`.

**Related Links**
https://github.com/Bukkit/Bukkit/pull/893
JIRA ticket: [Bukkit-4480](https://bukkit.atlassian.net/browse/BUKKIT-4480)