Blocks: Add the reusable block post type, `wp_block`.

Merges [43804] from the 5.0 branch to trunk.

See #45098.

Built from https://develop.svn.wordpress.org/trunk@44146


git-svn-id: https://core.svn.wordpress.org/trunk@43976 1a063a9b-81f0-0310-95a4-ce76da25c4cd