Add @api stable annotations for ol.coordinate

This one is questionable to me.  If there is not a solid use case for it currently, I'd be tempted to remove the `@api` annotation.  If it is being used in an application, I'd still be tempted to call it experimental.

My motivation here is that we have a huge API at this point, with a significant documentation (and build story) challenge.  I think we should be a bit less generous in what we export.