Checking if a field has a certain JSON type

I'd like to parse unstructured JSON. For example, a field that is either a string or []string. As a suggestion, I feel there is the need for `IsString`, `IsArray`, etc methods. These would theoretically allow checking the type of an AstNode in a performant way. Currently, I am using two techniques to accomplish this, both of which seem poor:

1. Call `String` or `Array` and check for errors
2. Call `Interface`, then use reflection to check the type

If an `IsString`, etc method would be no more performant than either of these techniques, feel free to ignore my suggestion.

Start by locating AstNode and its existing String, Array, and Interface methods, then inspect how their errors distinguish JSON types. Done means the requested string and array type checks are supported with performance behavior considered and coverage for the new API.

Written by the indexing model from the issue text.