Change name of Declaration node to be Identifier | ComputedPropertyName

This adds a type for ComputedPropertyName from ES6, and the necessary type modification in the compiler's Node hierarchy.
Ha, FunkyDeclaration! This actually includes FunctionExpression as well, so it is not limited to declarations. However, I see your point and I'll change it to FunctionLikeDeclaration. I will also add a comment with examples of things that should extend it.