Branches the given async iterable into the n branches.

Example:

    import { tee } from "./tee.ts";

    const gen = async function* gen() {
      yield 1;
      yield 2;
      yield 3;
    }

    const [branch1, branch2] = tee(gen());

    for await (const n of branch1) {
      console.log(n); // => 1, 2, 3
    }

    for await (const n of branch2) {
      console.log(n); // => 1, 2, 3
    }