REST API: Add missing options to the settings endpoint.

This adds the `show_on_front`, `page_on_front`, and `page_for_posts` options to the settings endpoint that were missed during WP 6.0 backports.

Related PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/38607 #38607 Page for Posts: Display notice in template panel]

Props Mamaduka, spacedmonkey, gziolo, jameskoster.
See #56058.

git-svn-id: https://develop.svn.wordpress.org/trunk@53588 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/option.php

@@ -2063,6 +2063,7 @@ function set_site_transient( $transient, $value, $expiration = 0 ) {
  * does not encompass all settings available in WordPress.
  *
  * @since 4.7.0
+ * @since 6.1.0 The `show_on_front`, `page_on_front`, and `page_for_posts` options were added.
  */
 function register_initial_settings() {
 	register_setting(
@@ -2220,6 +2221,36 @@ function register_initial_settings() {
 		)
 	);
 
+	register_setting(
+		'reading',
+		'show_on_front',
+		array(
+			'show_in_rest' => true,
+			'type'         => 'string',
+			'description'  => __( 'What to show on the front page' ),
+		)
+	);
+
+	register_setting(
+		'reading',
+		'page_on_front',
+		array(
+			'show_in_rest' => true,
+			'type'         => 'number',
+			'description'  => __( 'The ID of the page that should be displayed on the front page' ),
+		)
+	);
+
+	register_setting(
+		'reading',
+		'page_for_posts',
+		array(
+			'show_in_rest' => true,
+			'type'         => 'number',
+			'description'  => __( 'The ID of the page that should display the latest posts' ),
+		)
+	);
+
 	register_setting(
 		'discussion',
 		'default_ping_status',