Use .toBeExtensible when checking if an object is extensible

import {
  defineExpect,
  not,
  test,
  toBeExtensible,
} from "https://deno.land/x/unitest@$VERSION/mod.ts";

const expect = defineExpect({
  matcherMap: {
    toBeExtensible,
  },
  modifierMap: {
    not,
  },
});

test("passes when value is extensible", () => {
  expect({ a: 1 }).toBeExtensible();
  expect(1).not.toBeExtensible();
});