feat: support subshells

[dsherret]

{
  const result = await $`(echo 1 && echo 2) | cat -`.text();
  assertEquals(result, "1\n2");
}
{
  const result = await $`echo 1 && (echo 2 && exit 1 && echo 3) || echo 4`.text();
  assertEquals(result, "1\n2\n4");
}