[core] pushItem and removeItem methods on array field handle

[mixvar]

from POC:

      /**
       * Will set the array field value to its copy with the item added at the end.
       * Will run field validation with `change` trigger.
       * Will set `isTouched` to `true`.
       */
      pushItem: (item: E) => void;

      /**
       * Will set the array field value to its copy with the item at index `i` removed.
       * Will run field validation with `change` trigger.
       * Will set `isTouched` to `true`.
       */
      removeItem: (i: number) => void;