Add ipcInput option

[ehmicky]

This PR adds an ipcInput option, which allows passing almost any value to a subprocess. It sends that value using subprocess.sendMessage() under the hood.

As opposed to argv, environment variables and stdin, it is not limited to strings/Uint8Array. Any value that works with the structured clone algorithm works.

Unlike them though, it only works if the subprocess uses Node.js.

Example:

// main.js
import {execaNode} from 'execa';

const ipcInput = [
	{task: 'lint', ignore: /test\.js/},
	{task: 'copy', files: new Set(['main.js', 'index.js']),
}];
await execaNode({ipcInput})`build.js`;

// build.js
import {getOneMessage} from 'execa';

const ipcInput = await getOneMessage();

[sindresorhus]

Conflict

[ehmicky]

Fixed. 👍