fix: toString() on generator

[0xe]

toString() on a generator misses the *. This change adds that to the encodedSource.

For example,

  function* f() {
    yield 1;
  }; f.toString();

returns,

Before:

  function f() {
    yield 1;
  }

After:

  function* f() {
    yield 1;
  }

[rbri]

@0xe thanks for your PR