add convenience methods to stdio objects in `Deno.ChildProcess`

For instance, right now getting the `stdout` output as text is very non-obvious:

```ts
const sub = new Deno.Command("cat", {
  args: ["hello.txt"],
  stdout: "piped",
}).spawn();

const output = await new Response(sub.stdout).text();
```

it should be simpler, like:

```ts
const output = await sub.stdout.text();
```