Use AssetsLoader for enqueue css in formEditor

[MAILPOET-3556]

lib/Config/AssetsLoader.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace MailPoet\Config;
+
+use MailPoet\WP\Functions as WPFunctions;
+
+class AssetsLoader {
+
+  /** @var Renderer */
+  private $renderer;
+
+  /** @var WPFunctions */
+  private $wp;
+
+  public function __construct(RendererFactory $rendererFactory, WPFunctions $wp) {
+    $this->renderer = $rendererFactory->getRenderer();
+    $this->wp = $wp;
+  }
+
+  public function loadStyles(): void {
+    if (isset($_GET['page']) && $_GET['page'] === 'mailpoet-form-editor') {
+      $this->enqueueStyle('mailpoet-form-editor');
+      $this->enqueueStyle('mailpoet-public');
+    }
+  }
+
+  private function enqueueStyle(string $name): void {
+    $this->wp->wpEnqueueStyle(
+      $name,
+      Env::$assetsUrl . '/dist/css/' . $this->renderer->getCssAsset("{$name}.css")
+    );
+  }
+}