Add "ValidSkinNames" to extension.schema.json and ExtensionProcessor

skin.json goes through the same exact code as extension.json does, and needs
to be able to add skins.

Change-Id: Idd0acaf6735b4c308d3ab584c49da4e97448347f

docs/extension.schema.json

@@ -611,6 +611,9 @@
 		"APIListModules": {
 			"type": "object"
 		},
+		"ValidSkinNames": {
+			"type": "object"
+		},
 		"callback": {
 			"type": [
 				"array",

includes/registration/ExtensionProcessor.php

@@ -44,6 +44,7 @@ class ExtensionProcessor implements Processor {
 		'APIMetaModules',
 		'APIPropModules',
 		'APIListModules',
+		'ValidSkinNames',
 	);
 
 	/**