Change type annotation to mixed due to lack of guarantees on type

From usage, it very much looks like the type will always be int,
but on the off chance there is some code path where this gets called with
something else, type was widened to mixed for now, per disussion in #1088

src/Compile/Base.php

@@ -72,11 +72,11 @@ protected function formatParamsArray(array $_attr): array {
 	/**
 	 * Returns attribute index for unnamed ("shorthand") attribute, or null if not allowed.
 	 *
-	 * @param int $key
+	 * @param string|int|null $key Index of the argument. Type should probably be narrowed to int
 	 *
 	 * @return string|int|null
 	 */
-	protected function getShorthandOrder(int $key) {
+	protected function getShorthandOrder(mixed $key) {
 	    return $this->shorttag_order[$key] ?? null;
 	}
 

src/Compile/Tag/BCPluginWrapper.php

@@ -27,11 +27,11 @@ public function __construct($callback, bool $cacheable = true) {
          * For compiler plugins, we allow arbitrarily many unnamed attributes,
          * and just make them accessible in the order they are set.
          *
-         * @param int $key
+	 * @param string|int|null $key Index of the argument. Type should probably be narrowed to int
          *
-         * @return int
+         * @return string|int|null
          */
-        protected function getShorthandOrder(int $key): int {
+        protected function getShorthandOrder(mixed $key) {
             return $key;
         }