Add non-send resources via Commands

[willcrichton]

What problem does this solve or what need does it fill?

I have a non-send resource (RustPython interpreter). It has a sufficiently complicated construction process such that I didn't want to run the constructor in Plugin::build, but rather have my plugin launch a startup system that would insert the non-send resource.

Right now, the only way to insert a non-send (thread-local) resource is either via AppBuilder, or via a thread-local system that directly manipulates Resources.

What solution would you like?

Using the Commands struct provides a more consistent way of managing resources outside of AppBuilder. As proposed by RawMeatball here, the API could look like:

fn forced_non_send_system(
  commands: &mut Commands,
  marker: NonSend<()>,
) {
  let custom_thing = Stuff::that_is_non_send();
  commands.insert_non_send_resource(&marker, custom_thing);
}

What alternative(s) have you considered?

Could just leave it the way it is. This isn't a necessity, but a nice bit of API consistency.