fix(core): parse stdout in json() by default and allow source selection

[shubhransh-gupta]

Fixes #1505

Problem

ProcessOutput.json() previously parsed this.stdall. When CLI commands write structured JSON to stdout while logging diagnostic/progress messages to stderr (or when tools output logs to stderr), stdall contains interleaved text from both streams, causing SyntaxError: Unexpected token ... in JSON at position ....

Solution

• By default, ProcessOutput.json() now inspects this.stdout. If this.stdout.trim() is non-empty, it parses this.stdout.
• For backward compatibility (such as synthetic ProcessOutput objects initialized with empty stdout), it falls back to this.stdall.
• Added an optional source?: 'stdout' | 'stderr' | 'stdall' | { source?: 'stdout' | 'stderr' | 'stdall' } parameter to both ProcessPromise.json() and ProcessOutput.json(), allowing users to explicitly select which output stream to parse if desired.
• Added comprehensive unit tests in test/core.test.js and updated documentation in docs/process-promise.md.

import {$} from 'zx'

// When CLI prints logs to stderr and JSON to stdout:
const data = await $`node -e "console.error('downloading...'); console.log(JSON.stringify({ status: 'ok' }))"`.json()
console.log(data.status) // => 'ok'

// Explicit stream selection:
const stderrData = await $`...`.json('stderr')

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