Add multiplication function

[Tlacenka]

User story

As a user, I want to be able to multiply numbers as well, so that I don't have to add them up N times instead.

Acceptance criteria

• A multiplication function is implemented.
• Tests cover this new functionality.

Implementation details

Test case example:

test("should multiply several numbers", () => {
  expect(multiply([2, 3, 4])).toBe(24);
});