_post_states should include Scheduled

[ragulka]

It would make a lot of sense if _post_states would also output "Scheduled", if the post is scheduled to be published in the future. This way, the admin could easily see which posts in the post list are already published and which ones will be published by scheduling.

[rclations]

Added schedule & missed schedule based on similar work in Date column

[brandbrilliance]

Actually, I think all the post status should be available, but then a flag indicating which ones are visible and which ones are invisible, then we can turn then on/off in a plugin.

[helen]

I've been sitting on this for a bit just to see how it feels after paying attention, and I think it does make sense to add "Scheduled" even though it's also over in the date column. "Missed Schedule" is not something I would want to add, though, as it's a bit scarier, we don't put any other "advisory" type notices there, and we don't describe or really offer any fixes by default. That's not unique to this positioning, I just don't want to also add it here :)

[helen]

In 32902:

Add "Scheduled" to post states.

props rclations.
fixes #32131.