`ProcessOutput.json()` parses combined stdout+stderr, breaking on CLIs that log progress to stderr

### What happened?

`ProcessOutput.json()` runs `JSON.parse` on the combined output (`stdall`), not on stdout. Well-behaved CLIs write JSON to stdout and progress/prompts to stderr, so the stderr output gets mixed in and `json()` throws even though stdout contains valid JSON.

We hit this in [Wasp](https://github.com/wasp-lang/wasp), where we orchestrate the Railway CLI with its `--json` flag: `railway add --json` prints the JSON result to stdout but echoes the prompt answers to stderr, so `.json()` broke unexpectedly. We had to resort to parsing stdout ourselves with `JSON.parse(output.stdout)`: https://github.com/wasp-lang/wasp/pull/4711

### How it should work?

`json()` should parse stdout only. If the combined behavior is intentional, it would help to document which stream `json()` reads and offer a stdout-only variant e.g. `json({ source: 'stdout' })`

### How to reproduce the bug?

```js
import { $ } from 'zx'

const out = await $({ quiet: true })`node -e "console.error('progress...'); console.log('{\\"ok\\":true}')"`

JSON.parse(out.stdout) // { ok: true }
out.json() // SyntaxError: Unexpected token 'p', "progress.."... is not valid JSON
```

### Version

8.3.2

### What's OS kind?

Mac

Start at the ProcessOutput.json() implementation and run the supplied Node.js reproduction with stderr progress and JSON on stdout. Trace which output stream the method parses; done means valid stdout JSON parses successfully even when stderr contains progress text, with coverage for the reproduced case.

Written by the indexing model from the issue text.