Add DefineList.prototype.includes method
Fixes #277 by adding includes method to DefineList:
import {DefineList} from "can";
const list = new DefineList(["a", "b", "c"]);
console.log(list.includes("a")); // true
console.log(list.includes("foo")); // false
console.log(list.includes("c", -100)); // true
console.log(list.includes("b", 100)); // false