This changeset switches the `wp_enqueue_*` functions to `wp_register_*` for IE-related resources, which maintains handles, source references, etc., keeps all
IE-specific files within the theme package to avoid errors, and requires site admins to opt in to loading them in the theme.
It also replaces the content of `html5.js` shiv with a comment (to avoid 404s), removes IE-specific code in general stylesheets, and removes IE specific code
from Customizer, header.php and custom-header.php.
Props desrosj, sabernhardt, audrasjb, neychok, oglekler.
See #56699.
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