Add native block-editor support

• Add wp-scripts for build
• Enqueue script, excluding it from widget block editor
• Localize script with configurable/filterable options
• Localize script with titles for post types to match current functionality
• Register meta and set sanitization callbacks
• Allow meta to be accessed via endpoint
• Create sidebar plugin to show the two fields used by the plugin

inc/class-external-permalinks-redux-block-editor.php

 <?php
 /**
- * Block-editor integration
+ * Block editor support.
  *
  * @package External_Permalinks_Redux
  */
 
-namespace External_Permalinks_Redux;
-
-class Block_Editor {}
+/**
+ * Class Block_Editor.
+ */
+class External_Permalinks_Redux_Block_Editor {}