Implement a function squashObject(obj) that flattens a deeply nested object into a single level.
The new object's keys are formed by joining nested keys with a period (.).

Rules:

• If a property value is an object or array, flatten it recursively.
• Null and undefined values must still be included.
• Arrays should use their index as part of the key.
• Empty keys ("") should be skipped (treated as if that layer does not exist).

Examples

const object1 = {
  a: 5,
  b: 6,
  c: {
    f: 9,
    g: {
      m: 17,
      n: 3,
    },
  },
};
squashObject(object1);
// { a: 5, b: 6, 'c.f': 9, 'c.g.m': 17, 'c.g.n': 3 }

const object2 = { a: { b: null, c: undefined } };
squashObject(object2);
// { 'a.b': null, 'a.c': undefined }

const object3 = { a: { b: [1, 2, 3], c: ['foo'] } };
squashObject(object3);
// { 'a.b.0': 1, 'a.b.1': 2, 'a.b.2': 3, 'a.c.0': 'foo' }

const object4 = {
  foo: {
    '': { '': 1, bar: 2 },
  },
};
squashObject(object4);
// { foo: 1, 'foo.bar': 2 }