Code Modernization: Explicitly declare all properties in `Gettext_Translations`.

Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.

There are a number of ways to mitigate this:
* If it is an accidental typo for a declared property: fix the typo.
* For known properties: declare them on the class.
* For unknown properties: add the magic `__get()`, `__set()`, et al. methods to the class or let the class extend `stdClass` which has highly optimized versions of these magic methods built in.
* For unknown ''use'' of dynamic properties, the `#[AllowDynamicProperties]` attribute can be added to the class. The attribute will automatically be inherited by child classes.

In this case, the `$_nplurals` and `$_gettext_select_plural_form` properties, both being set in the class constructor, fall in the “known property” category.

Reference: [https://wiki.php.net/rfc/deprecate_dynamic_properties PHP RFC: Deprecate dynamic properties].

Follow-up to [10584], [12079], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916], [53935], [53936], [53937], [53938], [53942], [53945], [53948], [53949], [53952], [53953], [53954].

Props jrf, antonvlasenko, costdev.
See #56033.
Built from https://develop.svn.wordpress.org/trunk@53957


git-svn-id: http://core.svn.wordpress.org/trunk@53516 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pomo/mo.php

@@ -13,6 +13,11 @@ require_once __DIR__ . '/streams.php';
 if ( ! class_exists( 'MO', false ) ) :
 	class MO extends Gettext_Translations {
 
+		/**
+		 * Number of plural forms.
+		 *
+		 * @var int
+		 */
 		public $_nplurals = 2;
 
 		/**

wp-includes/pomo/translations.php

@@ -181,6 +181,21 @@ if ( ! class_exists( 'Translations', false ) ) :
 	}
 
 	class Gettext_Translations extends Translations {
+
+		/**
+		 * Number of plural forms.
+		 *
+		 * @var int
+		 */
+		public $_nplurals;
+
+		/**
+		 * Callback to retrieve the plural form.
+		 *
+		 * @var callable
+		 */
+		public $_gettext_select_plural_form;
+
 		/**
 		 * The gettext implementation of select_plural_form.
 		 *

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53956';
+$wp_version = '6.1-alpha-53957';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.