feat(core): allow adding items to not initialized collections

[B4nan]

Works for both 1:m and m:n collections.

const a = await orm.em.findOneOrFail(Author2, god.id);
console.log(a.books.isInitialized()); // false
const b2 = new Book2('Bible 2', a);
const b3 = new Book2('Bible 3', a);
a.books.add(b2, b3);
await orm.em.flush();