archwiki: Enable ParserFunctions extension

ParserFunctions allows for more versatile wiki templates, amongst other
functionality, and is shipped with MediaWiki since version 1.18. [1]

Included string functionality remains disabled, as is done on wikimedia
projects. [2]

[1] https://www.mediawiki.org/wiki/Extension:ParserFunctions
[2] https://phabricator.wikimedia.org/T8455#110965

roles/archwiki/templates/LocalSettings.php.j2

@@ -286,6 +286,8 @@ wfLoadExtension( 'CheckUser' );
 $wgGroupPermissions['sysop']['checkuser'] = true;
 $wgGroupPermissions['sysop']['checkuser-log'] = true;
 
+# ParserFunctions extension
+wfLoadExtension( 'ParserFunctions' );
 
 ##
 ## Temporary settings for maintenance