Allow differentiating between macro if and unless

[Blacksmoke16]

Feature Request

A MacroIf ASTNode represents an if statement within a macro:

{% if cond %}
  puts "Then"
{% else %}
  puts "Else"
{% end %}

However, the same node type is used to represent:

{% unless cond %}
  puts "Unless"
{% end %}

But based on the current API there is no way to tell the difference between an if and unless.

I propose we implement some way to be able to differentiate between the two types. Current ideas include:

1. Either have a dedicated MacroUnless node This would be a breaking change
2. Add some sort of boolean flag #is_unless? method to the current node type