Bootstrap/Load: Set `WP_DEBUG` to true by default on `development` environments.

Props johnbillion, kraftbj, Clorith, joostdevalk, dlh.
Fixes #33161.
Built from https://develop.svn.wordpress.org/trunk@48372


git-svn-id: https://core.svn.wordpress.org/trunk@48141 1a063a9b-81f0-0310-95a4-ce76da25c4cd