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.

git-svn-id: https://develop.svn.wordpress.org/trunk@53957 602fd350-edb4-49c9-b593-d223f7449a82

src/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;
 
 		/**

src/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.
 		 *