Suppress PhanUndeclaredProperty phan errors

For `\Parser->extTemplateStylesCache`, which is similar to Scribunto's
`\Parser->scribunto_engine`, which we also suppress.

Change-Id: If25dc88407ba33c09875577231a75b16d9cba8e7

.phan/config.php

@@ -1,3 +1,13 @@
 <?php
 
-return require __DIR__ . '/../vendor/mediawiki/mediawiki-phan-config/src/config.php';
+$cfg = require __DIR__ . '/../vendor/mediawiki/mediawiki-phan-config/src/config.php';
+
+$cfg['suppress_issue_types'] = array_merge(
+	$cfg['suppress_issue_types'],
+	[
+		// \Parser->extTemplateStylesCache
+		'PhanUndeclaredProperty',
+	]
+);
+
+return $cfg;