feat: add filter to $output before it is cached (#2910)

Add `timber/output/pre-cache` filter to hook into cache output before it's cached.

---------

Co-authored-by: Nicolas Lemoine <nico.lemoine@gmail.com>

src/ImageHelper.php

@@ -396,7 +396,7 @@ public static function get_sideloaded_file_loc($file)
 
         /**
          * Filters basename for sideloaded files.
-         * @since 2.0.1
+         * @since 2.1.0
          * @example
          * ```php
          * // Change the basename used for sideloaded images.

src/Loader.php

@@ -176,6 +176,17 @@ public function render($file, $data = null, $expires = false, $cache_mode = self
 
             $template = $twig->load($file);
             $output = $template->render($data);
+
+            /**
+             * Filters $output before it is cached.
+             *
+             * @since 2.1.0
+             *
+             * @param string $output
+             * @param array  $data
+             * @param string $file
+             */
+            $output = \apply_filters('timber/output/pre-cache', $output, $data, $file);
         }
 
         if (false !== $output && false !== $expires && null !== $key) {