Deprecate/remove execa.stdout() and execa.stderr()

[ehmicky]

execa.stdout() and execa.stderr() are easy enough to reproduce with destructuring:

const { stdout } = await execa(...)

Removing those methods would make the API simpler, and reduce code size.

One potential benefit of execa.stdout() and execa.stderr() would be to pass ignore to the other standard output stream, which would make I/O faster. This is not implemented at the moment. This seems to me that this would be a micro-optimization, and easy enough to emulate by letting users explicitly pass that option.

Should those methods be removed or deprecated?

If so, I can submit a PR.

[sindresorhus]

Yes, let's do it.

[ehmicky]

Done at #234