remove mixed typehint to comply with PHP 7.4 and 7.3.

Is anyone still using those?

src/Primitive.php

@@ -175,7 +175,7 @@ public function equals(Primitive $value): bool
      * @param  mixed  $value
      * @return bool
      */
-    public function matches(mixed $value): bool
+    public function matches($value): bool
     {
         if ($value instanceof self) {
             $value = $value->get();