feat: add configurable output stream

[nightnei]

picospinner currently writes all spinner frames to process.stdout. That is fine for simple interactive CLIs, but it breaks commands that reserve stdout for machine-readable output, such as JSON intended for jq, shell scripts, or CI tooling.

This PR adds an optional stream field to DisplayOptions, so callers can route spinner UI to stderr:

  const spinner = new Spinner({
    text: 'Loading...',
    stream: process.stderr,
  });

This gives CLI authors a way to follow the common convention (https://clig.dev/#the-basics) that stdout is for program output and stderr is for status/progress UI, without monkeypatching picospinner internals (as I temporary did here).
The default behavior is unchanged: spinners still write to process.stdout unless a stream is provided.

Testing instructions

1. Let's assert that no regressions:

node --input-type=module -e "
  import { Spinner } from './dist/index.js';

  const spinner = new Spinner(
    { text: 'Loading...' },
    { colors: false }
  );

  spinner.start(80);

  setTimeout(() => {
    spinner.succeed('Done');
    console.log(JSON.stringify({ ok: true }));
  }, 250);
  "

2. Assert that you see:
   
3. Try the same with stderr:

node --input-type=module -e "
  import { Spinner } from './dist/index.js';

  const spinner = new Spinner(
    { text: 'Loading...', stream: process.stderr },
    { colors: false }
  );

  spinner.start(80);

  setTimeout(() => {
    spinner.succeed('Done');
    console.log(JSON.stringify({ ok: true }));
  }, 250);
  "

4. Assert that you see the same:
   
5. Try using jq to assert that it's indeed printed to stderr:
   

node --input-type=module -e "
  import { Spinner } from './dist/index.js';

  const spinner = new Spinner(
    { text: 'Loading...', stream: process.stderr },
    { colors: false }
  );

  spinner.start(80);

  setTimeout(() => {
    spinner.succeed('Done');
    console.log(JSON.stringify({ ok: true }));
  }, 250);
  " | jq

6. Assert that you see no errors and json is parsed well (previously you would get error jq: parse error: Invalid numeric literal at line 1, column 3):
   

@43081j @PondWader

[PondWader]

Thanks, in hindsight defaulting to stdout was a mistake. Next major we should switch the default.