feat: support --if-private and --if-public

[ForbesLindesay]

I'm not sure if this is useful or not. After writing I realised that you also need to check that the version numbers are up to date, which can be done using https://github.com/azu/can-npm-publish

In case it is of use. I'll leave this PR here.

[spion]

Not entirely sure but I believe wsrun --if can-npm-publish publish would work here, since can-npm-publish also checks the private field?

[ForbesLindesay]

That didn't work because yarn seems to run with an odd environment which means can-npm-publish does not manage to check the version number of the published package when it's called as a yarn script. It fails to authenticate properly with the npm registry. This PR doesn't fix that problem anyway, so feel free to close.

[joshgummersall]

@ForbesLindesay I published a tiny package '@standardlabs/is-private' that provides two commands, 'is-private' and 'is-not-private', both just look at the private field in a package file.