pick method
Returns a parser that transforms a successful parse result by returning
the element at index of a list. A negative index can be used to access
the elements from the back of the list.
For example, the parser letter().star().pick(-1) returns the last
letter parsed. For the input 'abc' it returns 'c'.
Implementation
@useResult
Parser<R> pick(int index) => PickParser<R>(this, index);