Set ALLOW_EXPERIMENT_REREGISTRATION to false in WebPack build.

Gutenberg introduced a system of sharing private APIs in
WordPress/gutenberg#46131. One of the
safeguards is a check preventing the same module from opting-in
twice so that contributors cannot easily gain access by pretending
to be a core module.

That safeguard is only meant for WordPress core and not for
the released `@wordpress` packages. However, right now it is
opt-out and must be explicitly disabled by developers wanting to
install the `@wordpress` packages. Let's make it opt-out instead.

This commit opts-out from that check in WordPress core by setting
the ALLOW_EXPERIMENT_REREGISTRATION to false. Once it's merged,
the Gutenberg plugin should be adjusted to use `true` as the
default value.

src/wp-includes/blocks/template-part.php

@@ -22,7 +22,7 @@ function render_block_core_template_part( $attributes ) {
 	if (
 		isset( $attributes['slug'] ) &&
 		isset( $attributes['theme'] ) &&
-		get_stylesheet() === $attributes['theme']
+		wp_get_theme()->get_stylesheet() === $attributes['theme']
 	) {
 		$template_part_id    = $attributes['theme'] . '//' . $attributes['slug'];
 		$template_part_query = new WP_Query(