Copy Validator to email-editor package

Because the email-editor package should be independent. We copy Validator to the package, at least for now.
[MAILPOET-6216]

packages/php/email-editor/src/Validator/Schema/ArraySchema.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\EmailEditor\Validator\Schema;
+
+use MailPoet\EmailEditor\Validator\Schema;
+
+// See: https://developer.wordpress.org/rest-api/extending-the-rest-api/schema/#arrays
+class ArraySchema extends Schema {
+  protected $schema = [
+    'type' => 'array',
+  ];
+
+  public function items(Schema $schema): self {
+    return $this->updateSchemaProperty('items', $schema->toArray());
+  }
+
+  public function minItems(int $value): self {
+    return $this->updateSchemaProperty('minItems', $value);
+  }
+
+  public function maxItems(int $value): self {
+    return $this->updateSchemaProperty('maxItems', $value);
+  }
+
+  public function uniqueItems(): self {
+    return $this->updateSchemaProperty('uniqueItems', true);
+  }
+}