Add `thread` option

[ehmicky]

Being able to spawn a worker thread instead of a child process using a simple boolean option would be nice.

await execa(..., {thread: true});

Unlike Worker in browsers, Node.js worker_threads includes many similar features as child processes: environment variables, stdin/stdout/stderr, argv.

Also, most of Execa's logic is actually userland, i.e. implemented by us and not borrowed from child_process.spawn(). For example, most things input/output-related (including subprocess piping), the verbose mode, etc. It would be nice to be able to re-use all of those Execa features but with a worker thread instead. I believe 90-95% of features can be re-used.

This option would imply the node: true option, since only Node.js files can be launched as threads.

Instead of returning a .subprocess property (#413), this would return a .worker property, with the Worker instead. We would try to make the properties/methods as close to .subprocess as possible, for parity.

That's a bigger feature, but this might be quite useful. What are your thoughts on this @sindresorhus?

[sindresorhus]

This sounds useful indeed. I am a little bit worried about the growing complexity, but if we really can reuse most of the code, it's not too bad.

However, I think it would be better to wait a bit for this, to ensure v9 is stable first.

[ehmicky]

Sounds good. 👍