REST API: Fix handling of some orderby parameters for the Posts controller.

- `'orderby' => 'include'` requires an array of post_ids via the `include` collection param.
`'orderby' => 'id'` and `'orderby' => 'slug'` need map the correct WP_Query equivalents. 

Props flixos90, hnle, dd32, rachelbaker, joehoyle, pento.

Fixes #38971.

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git-svn-id: https://core.svn.wordpress.org/trunk@39380 1a063a9b-81f0-0310-95a4-ce76da25c4cd