set & flatKeys | better return type | stricter input type

[Maggi64]

With tools like hotscript or advanced TS magic we might be able to improve the return type and validate the given path on compile time.

It should be:

const obj = { a: 1 };
const newObj = set(obj, 'a.c', 1);
//      ^?  { a: { c: number } }

Instead of the current state:

const obj = { a: 1 };
const newObj = set(obj, 'a.c', 1);
//      ^?  PlainObject