Change system set builder to force users to put the "config" first

[hymm]

What solution would you like?

with_system on the SystemSet builder returns something that only implements with_system, so config things like labeling, run_criteria, etc. must be placed before the systems.

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

This would fix #1912.

This would explicitly enforce @cart's preference on this comment #1909 (comment)

What alternative(s) have you considered?

We can just keep things as they are. Cargo fmt disambiguates this decently.

[Ratysz]

I prefer this style as well, but outright forbidding alternatives seems needlessly restrictive. Not to mention the small but non-negligible API complication that would be necessary.

[cart]

Agreed. Adopting the "type state" approach here would over-complicate the api and adopting an internal "state tracker" also feels pretty messy.

I'd rather just encourage it as a "best practice" in official examples.

[cart]

(feel free to continue discussing if you want. I'll reopen if any major dissenting opinions show up)