REST API: Fixes /wp/v2/pattern-directory/patterns endpoint response for slug parameter.

[53218] introduced a bug of a wrong response from the `wp/v2/pattern-directory/patterns` endpoint with a `slug` parameter. As the response is cached, it can result in an incorrect list of available patterns supported by the current theme.

This commit resolves by:

* Limiting the `slug` to an `array` in the query parameters.
* When set, parsing and sorting the slug(s) and then serializing the sorted query args as part of the hashed transient keys.

Props antonvlasenko, timothyblynjacobs, spacedmonkey, costdev, hellofromTonya.

Follow-up to [53218], [53152], [51208].
Fixes #55617.

git-svn-id: https://develop.svn.wordpress.org/trunk@53333 602fd350-edb4-49c9-b593-d223f7449a82

tests/qunit/fixtures/wp-api-generated.js

@@ -10385,6 +10385,11 @@ mockedApiResponse.Schema = {
                             "type": "integer",
                             "minimum": 1,
                             "required": false
+                        },
+                        "slug": {
+                            "description": "Limit results to those matching a pattern (slug).",
+                            "type": "array",
+                            "required": false
                         }
                     }
                 }