Add NumberLiteral#zero?

[Sija]

re: #10230 (comment)

[asterite]

I really don't see a need for this. Let's just remove the unused method for now.

[straight-shoota]

I agree to remove it. With #10462 you can just use {% number.to_i == 0%}. In the macro language we don't need a convenient short cut for that.

[straight-shoota]

With #10802 merged, the workaround proposed in my previous comment does not exactly apply: NumberLiteral#to_number returns 0 or 0.0 depending on whether it's an int or float literal. So equality with zero is not as trivial. But unless there is a reasonable use case, I think this should be closed.