Implement MSETEX and refactor SET.

* Implement the new `MSETEX` Redis command.
* Refactor parsing of extended `SET` arguments into unified helper
  functions, deduplicating logic.
* Add new `SET` arguments `IFNE`, `IFDEQ`, and `IFDNE`. We already
  support Valkey's existing `IFEQ` argument, which Redis has added.

Fixes #2742

redis.stub.php

@@ -2902,6 +2902,19 @@ class Redis {
      */
     public function mset(array $key_values): Redis|bool;
 
+    /**
+     * Set one or more keys and values with optional expiry information.
+     *
+     * @param array      $key_vals An array of keys with their values.
+     * @param int|float|array|null $expiry   An optional array with expiry information.
+     * @return Redis|int|false 1 if all keys were set, 0 if none were.
+     *
+     * @see https://redis.io/commands/msetex
+     *
+     * @example $redis->msetex(['foo' => 'bar', 'baz' => 'bop'], ['EX' => 60]);
+     */
+    public function msetex(array $key_vals, int|float|array|null $expiry = null): Redis|int|false;
+
     /**
      * Set one or more string keys but only if none of the key exist.
      *