Posts, Post Types: Add default `menu_icon` values for built-in post types.

This allows the icons to be retrieved via `get_post_type_object()`.

Props m.usama.masood, michael.ecklund, SergeyBiryukov.
Fixes #38844.
Built from https://develop.svn.wordpress.org/trunk@47196


git-svn-id: https://core.svn.wordpress.org/trunk@46996 1a063a9b-81f0-0310-95a4-ce76da25c4cd