Allow psalm to load threads from the config

src/Psalm/Config.php

@@ -563,6 +563,9 @@ class Config
      */
     public $internal_stubs = [];
 
+    /** @var ?int */
+    public $threads;
+
     protected function __construct()
     {
         self::$instance = $this;
@@ -1222,6 +1225,10 @@ class Config
             }
         }
 
+        if (isset($config_xml->threads)) {
+            $config->threads = (int)$config_xml->threads;
+        }
+
         return $config;
     }