Adding one object structure per file

[dingo-d]

This addition will allow WordPress/WordPress-Coding-Standards#1802 to be merged to the coding standards.

This will encourage having one class/interface/trait per file.

Feel free to suggest better wording, I tried to be as clear as possible.

[jrfnl]

For the record - this is how it was announced in the blog post about coding standards on Make last year:

One class-like definition per file

Proposed standards:

• There should only be one class-like definition (class/trait/interface) per file.

This has already been a best practice for a while and will now be formalized.

[jrfnl]

@dingo-d Thanks for doing this! Text looks good to me.

It does feel a bit "off" for this to be lumped in into the "Naming Conventions" section though, so I suggest adding it as its own section just below that one.

Other than that, just some punctuation fixes to be made and it is good to go.

[ntwb]

Looks good to me, add @jrfnl suggestions and feel free to merge

[dingo-d]

Thanks for the suggestions 👍🏼 I wasn't sure if this should have its own subchapter or not, so I just placed them at the end of the naming part, because that's closest to the issue (the way files should be named and all). But it does make more sense to be on its own.

I've committed the changes you've suggested, so this can be merged 🙂