Improve default value of objectMode option

[ehmicky]

Transforms can return objects by setting objectMode: true.

const transform = function * (line) {
	yield JSON.parse(line);
};

const {stdout} = await execa('./jsonlines-output.js', {stdout: {transform, objectMode: true}});
for (const data of stdout) {
	console.log(stdout); // {...}
}

The default value for objectMode is false. However, when multiple transforms are used, and the previous transform uses objectMode: true, the next one should default to true instead. For example:

await execa('./jsonlines-output.js', {stdout: [
  {transform, objectMode: true}, 
  otherTransform,
]});

otherTransform should be in objectMode since the previous does. It can still revert to non-objectMode by doing:

await execa('./jsonlines-output.js', {stdout: [
  {transform, objectMode: true}, 
  {transform: otherTransform, objectMode: false},
]});

This enables creating re-usable transforms that work regardless of whether the previous transform is in objectMode or not. We already use internally some of those objectMode-agnostic transforms, which work regardless of whether chunks are objects or not, and that should keep the objectMode as is. Without the above behavior, every transform must choose either true or false for objectMode and cannot be truly agnostic to it.