Toolbar: Load toolbar in `wp_body_open` when available.

For accessibility, the visual appearance and source order should match. Moving the toolbar to load in the new hook `wp_body_open` (5.2) fixes a long-standing source order problem.

Props jankimoradiya, afercia, SergeyBiryukov, audrasjb, ocean90, xkon, dinhtungdu.
Fixes #47053.

Built from https://develop.svn.wordpress.org/trunk@47221


git-svn-id: https://core.svn.wordpress.org/trunk@47021 1a063a9b-81f0-0310-95a4-ce76da25c4cd