Script Loader: Harden removal of script tag wrappers.

* Add `wp_remove_surrounding_empty_script_tags()` to more precisely remove script tag wrappers and warn when doing it wrong.
* Add clarifying comments for XML escaping logic in `wp_get_inline_script_tag()`.
* Leverage `WP_HTML_Tag_Processor` in `test_remove_frameless_preview_messenger_channel`.
* Reuse `assertEqualMarkup` in `test_blocking_dependent_with_delayed_dependency`.
* Normalize whitespace in `parse_markup_fragment` for `assertEqualMarkup`.

Follow-up to [56687].
Props dmsnell, westonruter, flixos90.
See #58664.

Built from https://develop.svn.wordpress.org/trunk@56748


git-svn-id: http://core.svn.wordpress.org/trunk@56260 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -3800,7 +3800,7 @@ function wp_customize_support_script() {
 		}());
 	</script>
 	<?php
-	wp_print_inline_script_tag( str_replace( array( '<script>', '</script>' ), '', ob_get_clean() ) );
+	wp_print_inline_script_tag( wp_remove_surrounding_empty_script_tags( ob_get_clean() ) );
 }
 
 /**