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.
I like the concept, but I'm not crazy about the name `FunctionLike`. What gets lost is that it is a declaration (vs. say a symbol or a type). Perhaps FunctionLikeDeclaration is better. Or FunkyDeclaration. :-)