Add Token::Kind#unary_operator?

[straight-shoota]

This is a tiny refactor to merge two duplicate definitions of unary operators into a predicate method of Token::Kind.
Also simplifies the handling of unary operators in prefix notation in the formatter.

There's another similar definition in ToSVisitor::UNARY_OPERATORS but it's based on method name, not token type. This is practically not very relevant, but it's not trivial to integrate into one. So I think it's fine to keep this duplication, but I've added cross-references between the two.

[HertzDevil]

I think the integration would just be:

module Crystal
  class ToSVisitor
    def visit(node : Not)
      visit_call Call.new(node.obj, "!")
    end
  end
end

Not the fastest code ever, but the formatter does a similar thing all over the place (e.g. as and sizeof).

[straight-shoota]

I don't think Not is an issue with ToSVisitor. We could do what you suggested, or not and just leave it as is (which should be totally fine).

The challenge is that there's no token in ToSVisitor.