Introduce native Gutenberg controls

• Register meta
• Enqueue script
• Switch to use_block_editor_for_post_type and remove the option
• Build component
• Add sanitize callback to handle special cases
• Add auth callback to allow "private" meta to be edited
• Add padding between modal buttons
• Don't hide UI if set to 0, as there's no way to update the setting
• Switch to @wordpress/scripts
• Consider moving Gutenberg bits, and/or REST API elements, to their own class.

README.md

 # WP Revisions Control #
-**Contributors:** ethitter  
-**Donate link:** https://ethitter.com/donate/  
-**Tags:** revision, revisions, admin  
-**Requires at least:** 3.6  
-**Tested up to:** 5.7  
-**Stable tag:** 1.4  
-**License:** GPLv2 or later  
-**License URI:** http://www.gnu.org/licenses/gpl-2.0.html  
+**Contributors:** ethitter
+**Donate link:** https://ethitter.com/donate/
+**Tags:** revision, revisions, admin
+**Requires at least:** 3.6
+**Tested up to:** 5.9
+**Stable tag:** 1.4
+**License:** GPLv2 or later
+**License URI:** http://www.gnu.org/licenses/gpl-2.0.html
 
 Control how many revisions are stored for each post type.
 
@@ -73,4 +73,3 @@ For any post type that supports revisions, you can now limit the number of revis
 
 ### 2. The post-level controls provided in version 1.2. ###
 ![The post-level controls provided in version 1.2.](http://ps.w.org/wp-revisions-control/assets/screenshot-2.png)
-