Add wp-packages-update command to update WordPress packages.

This ticket exposes the packages-update script implemented at WordPress/gutenberg#19448 on WordPress core.
By running "npm run wp-packages-update" all the WordPress packages are updated to the latest version. The command is very useful when updating the Gutenberg editor core exposes.

Props isabel_brison.
Fixes #51491.
Built from https://develop.svn.wordpress.org/trunk@49129

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-49128';
+$wp_version = '5.6-alpha-49129';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.