Simplify passing options

[ehmicky]

This PR is a proposal to allow users to pass node:child_process native options directly at the top-level instead of namespacing them in options.nativeOptions.

Using options.nativeOptions is nice as it allows us to distinguish between nano-spawn curated options, and forwards "everything else" to node:child_process spawn().

That being said, I believe allowing native options at the top level could provide with more pros than cons:

1. The most commonly used options are most likely going to be some of the native ones: stdout: 'inherit', timeout, signal, shell, env, cwd. Entering {nativeOptions: {timeout: 1000}} is more cumbersome than directly {timeout: 1000}.
2. Exposing some of the native options at the top level (like we currently do with signal and timeout), but not others, requires typing and documenting them. It also adds a few bytes (although not much). Flattening it the top level would remove that, keeping the API and types small. Hopefully, nano-spawn will not have that many options (if any? let's see), so it'd be nice to be able to keep the API very small.

Please let me know what you think.

[sindresorhus]

Yeah, I agree. It's probably easier this way.