Support unions in Crystal::Macros::ASTNode#is_a?

[HertzDevil]

Implements part of #11981. This now works:

macro foo(x)
  {{ x.is_a?(NumberLiteral | StringLiteral) }}
end

foo(1)    # => true
foo("a")  # => true
foo(true) # => false

It contains a rather basic implementation of the type system of the macro language, supporting unions and NoReturn. Generics will be added later. There is a lot of room for optimizing the unions, but they can only be created through explicit |s so the impact is probably low.

As a reminder, ASTNode#class_name is never affected by this type system, whether that AST node is documented or not. Thus Crystal::Program#define_macro_types should define everything that may possibly appear within the macro language (#3274 (comment)), so that things like {{ x.is_a?(ClassDef) }} do not break afterwards. Everything else (e.g. Int32) implicitly names the macro NoReturn.

[beta-ziliani]

Looks great to me!