Add support for unary + and - operators

[jg-rp]

Add support for unary + and - operators.

require "liquid2"

env = Liquid2::Environment.new(arithmetic_operators: true)

source = <<~LIQUID.chomp
  {{ -(1+2) }}
  {{ -1+2 }}
  {{ -"42" }}
  {{ +"42" }}
LIQUID

puts env.render(source)

-3
1
-42
42

If the operand can't be coerced to a number following the same semantics as math filters, an instance of Liquid::Undefined is used instead.