Plurals are not exported correctly when exporting source translations

Recap from DrupalCon Lille 2023:

The functionallity of the latest patch #37 📌 Plurals are not exported correctly when exporting source translations Needs work looks fine. If the reroll of the patch passes, we put this back to Needs Review.

Moved last patch to MR and fixed following to make the test case to pass

- https://www.drupal.org/node/3168858 →
- https://www.drupal.org/node/3129738 →

NW to add

1. +++ b/core/lib/Drupal/Component/Gettext/PoItem.php
   @@ -49,6 +49,13 @@ class PoItem {
   +   * @var int
   ...
   +  protected $pluralCount;
   
   +++ b/core/lib/Drupal/Component/Gettext/PoStreamReader.php
   @@ -62,6 +62,13 @@ class PoStreamReader implements PoStreamInterface, PoReaderInterface {
   +   * @var int
   ...
   +  protected $pluralCount = 2;
   
   +++ b/core/lib/Drupal/Component/Gettext/PoStreamWriter.php
   @@ -35,6 +35,13 @@ class PoStreamWriter implements PoWriterInterface, PoStreamInterface {
   +   * @var int
   ...
   +  protected $pluralCount = 2;
   
   +++ b/core/modules/locale/src/PoDatabaseReader.php
   @@ -34,6 +34,13 @@ class PoDatabaseReader implements PoReaderInterface {
   +   * @var int
   ...
   +  private $pluralCount = 2;
   

   should be typed to int, so default value will be 0

2. +++ b/core/lib/Drupal/Component/Gettext/PoPluralCountAwareInterface.php
   @@ -0,0 +1,28 @@
   +   * @return self
   ...
   +  public function setPluralCount(int $pluralCount);
   
   +++ b/core/lib/Drupal/Component/Gettext/PoStreamReader.php
   @@ -126,6 +133,21 @@ public function getHeader() {
   +  public function setPluralCount(int $pluralCount) {
   
   +++ b/core/lib/Drupal/Component/Gettext/PoStreamWriter.php
   @@ -75,6 +82,21 @@ public function setLangcode($langcode) {
   +  public function setPluralCount(int $pluralCount) {
   

   Probably should return :self as of PHP 8.0 https://php.watch/versions/8.0/static-return-type