Add comment count to "replies" link in post responses

[rmccue]

Right now, you can preload comments using embedding with the linked "replies" resource. This is great, but there's no way to know whether you should try loading more. Fetching the resource directly would give you the total in the X-WP-Total header, but you lose this when embedding.

While this is generically a problem with the loss of headers from embedded resources, I don't think we really need a generic solution. Even when not embedding, a comment count is useful for archive display of a list of posts.

We should add the comment count (using wp_count_comments) to the "replies" link:

{
    "replies": [
        {
            "href": ".../wp/v2/comments?post=42",
            "embeddable": true,
            "count": 12,
        }
    ]
}

[wongalex]

I can look at this as my first bug :)

[TimothyBlynJacobs]

A generic solution to this problem does exist, targetHints, which I would love for us to support. ​https://json-schema.org/draft/2019-09/json-schema-hypermedia.html#rfc.section.6.5.5