Script Loader: Add a temporary `jquery-migrate-compat-5.5` script handle.

This is intended as an easy way for plugins and themes to still be able to load jQuery with jQuery Migrate in WordPress 5.5.

Once jQuery is updated to 3.x, this temporary script handle will be removed. This means plugins and themes still using it at that point will fail to load their scripts (as a dependency will be missing), but will not cause "fatal errors" breaking the rest of JS.

Follow-up to [48323], [48324].

Props azaozz, desrosj, apedog, Clorith, jorbin.
Fixes #37110.
Built from https://develop.svn.wordpress.org/trunk@48753


git-svn-id: http://core.svn.wordpress.org/trunk@48515 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/script-loader.php

@@ -723,6 +723,11 @@ function wp_default_scripts( $scripts ) {
 	$scripts->add( 'jquery-core', '/wp-includes/js/jquery/jquery.js', array(), '1.12.4-wp' );
 	$scripts->add( 'jquery-migrate', "/wp-includes/js/jquery/jquery-migrate$suffix.js", array(), '1.4.1' );
 
+	// Backward compatibility: Makes it easy for plugins and themes to enqueue jQuery with jQuery Migrate in WordPress 5.5.
+	// Note: This script handle will most likely be removed in WordPress 5.6 or when jQuery is updated to the latest version.
+	// Then plugins and themes that are still relying on it will stop functioning.
+	$scripts->add( 'jquery-migrate-compat-5.5', false, array( 'jquery-core', 'jquery-migrate' ), '1.12.4-wp' );
+
 	// Full jQuery UI.
 	$scripts->add( 'jquery-ui-core', "/wp-includes/js/jquery/ui/core$dev_suffix.js", array( 'jquery' ), '1.11.4', 1 );
 	$scripts->add( 'jquery-effects-core', "/wp-includes/js/jquery/ui/effect$dev_suffix.js", array( 'jquery' ), '1.11.4', 1 );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-48751';
+$wp_version = '5.6-alpha-48753';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.