MDL-72658 lang: Let welcome message use other than just firstname

The patch allows for strings welcomeback and welcometosite to be
customised and use other user names than just the firstname.

Supported are placeholder properties matching all known name fields:
firstnamephonetic, lastnamephonetic, middlename, alternatename,
firstname and lastname. Special values fullname and alternativefullname
are supported, too.

By default, the firstname is kept for compatibility with the original
feature design.

lang/en/moodle.php

@@ -2266,8 +2266,8 @@
 $string['weekhide'] = 'Hide this week from {$a}';
 $string['weeklyoutline'] = 'Weekly outline';
 $string['weekshow'] = 'Show this week to {$a}';
-$string['welcomeback'] = 'Welcome back, {$a}! 👋';
-$string['welcometosite'] = 'Welcome, {$a}! 👋';
+$string['welcomeback'] = 'Welcome back, {$a->firstname}! 👋';
+$string['welcometosite'] = 'Welcome, {$a->firstname}! 👋';
 $string['welcometocourse'] = 'Welcome to {$a}';
 $string['welcometocoursetext'] = 'Welcome to {$a->coursename}!
 

lib/classes/user.php

@@ -1189,7 +1189,17 @@ public static function welcome_message(): ?lang_string {
                 $messagekey = 'welcometosite';
                 set_user_preference('core_user_welcome', time());
             }
-            return new lang_string($messagekey, 'core', $USER->firstname);
+
+            $namefields = [
+                'fullname' => fullname($USER),
+                'alternativefullname' => fullname($USER, true),
+            ];
+
+            foreach (\core_user\fields::get_name_fields() as $namefield) {
+                $namefields[$namefield] = $USER->{$namefield};
+            }
+
+            return new lang_string($messagekey, 'core', $namefields);
         };
         return null;
     }