Add hasVariable to binding

Currently, if you want to safely check whether a variable exists in the binding or not, you have to use code like binding.variables?.containsKey('prop'), which is unintuitive. So this pull request adds a hasVariable() method, which is the one that I automatically attempt to use before realising it doesn't exist.

Erm...this pull request done completely via GitHub UI, so may need testing

cf https://github.com/groovy/groovy-core/pull/17