Add method list field from template (#4)

* Update src/PHPJasper.php

Co-authored-by: Josue <josueperezn2901@gmail.com>

Author: eliurkis

src/PHPJasper.php

@@ -210,6 +210,22 @@ protected function validateFormat($format)
         }
     }
 
+    /**
+     * @param string $input
+     * @return mixed
+     * @throws Exception\InvalidInputFile
+     */
+    public function listFields(string $input)
+    {
+        if (!is_file($input)) {
+            throw new Exception\InvalidInputFile();
+        }
+
+        preg_match_all("/<field name=\"(.*?)\"/s", file_get_contents($input), $matches);
+
+        return $matches[1] ?? [];
+    }
+
     /**
      * @param string $input
      * @return $this