Block Editor: Fix loading the script handling the "Manage Reusable Blocks" page.

This ensures the "import from JSON" and "export JSON" buttons are shown properly. The script is loaded in the footer because it relies on the DOM being already there.

Props noisysocks, mukesh27.

Merges [43936] into trunk.

Fixes #45396.
Built from https://develop.svn.wordpress.org/trunk@44274


git-svn-id: https://core.svn.wordpress.org/trunk@44104 1a063a9b-81f0-0310-95a4-ce76da25c4cd