Add isBotUser() and isRegularUser() to MessageAuthor

[Saladoc]

No description provided.

[Vampire]

Hm, I remember we talked about this method previously (Bastian and I afair, but in some channel) and came to the conclusion not to add it, but I don't remember the reasoning.

[Bastian]

The reason was, that it might cause even more confusion with the message author class:

• If isBot() is false someone might think that isUser() is true then.
• If isUser() is false someone might think that isBot is true then.

Maybe something like isBotUser() and isHumanUser() might be possible.

[Vampire]

I think we also considered those and decided against them. :-D

[Saladoc]

Not having a method to check if a MessageAuthor is a bot account is just plain counterintuitive though as it is one of the most common use cases so we should include some way.

I'd probably do all three isUser(), isBotUser(), and isHumanUser(), although I dislike the latter name.

[Vampire]

Please read https://discordapp.com/channels/151037561152733184/354648262453624833/467999839683215372 until 17.07. for reference.