Add Container class

This class is used as DI Container for easier injecting classes in integration tests.
It can be also reused during a standalone usage.
[MAILPOET-6216]

packages/php/email-editor/tests/unit/EmailEditor/ContainerTest.php

@@ -0,0 +1,44 @@
+<?php declare(strict_types = 1);
+
+use MailPoet\EmailEditor\Container;
+use PHPUnit\Framework\TestCase;
+
+class ContainerTest extends TestCase {
+  public function testSetAndGetService(): void {
+    $container = new Container();
+
+    $container->set('simple_service', function () {
+      return new stdClass();
+    });
+
+    $service = $container->get('simple_service');
+
+    $this->assertInstanceOf(stdClass::class, $service);
+  }
+
+  public function testGetReturnsSameInstance(): void {
+    $container = new Container();
+
+    $container->set('singleton_service', function () {
+      return new stdClass();
+    });
+
+    // Retrieve the service twice
+    $service1 = $container->get('singleton_service');
+    $service2 = $container->get('singleton_service');
+
+    // Check that both instances are the same
+    $this->assertSame($service1, $service2);
+  }
+
+  public function testExceptionForNonExistingService(): void {
+    // Create the container instance
+    $container = new Container();
+
+    // Attempt to get a non-existing service should throw an exception
+    $this->expectException(Exception::class);
+    $this->expectExceptionMessage('Service not found: non_existing_service');
+
+    $container->get('non_existing_service');
+  }
+}