Use extend theme attribute and add theme config

Add the extend attribute to the theme info via the theme info comment
block rather than adding it in the volse_init() method. This also makes
the volse_init() method obsolete for now.

See MR 2217:
https://framagit.org/hubzilla/core/-/merge_requests/2217

Also add a theme config class that extends the redbasic config class.
This enables the extra theme configuration and schema support from
Redbasic.

volse/php/theme.php

@@ -1,17 +1,14 @@
 <?php
-/**
+/*
  * Name: Volse
  * Description: Default theme for Volse Hubzilla
  * Version: 1.0
  * Author: Harald Eilertsen
  * Compat: Redbasic
+ * Extends: redbasic
  *
  * SPDX-FileCopyrightText: 2025 Eilertsens Kodeknekkeri
  * SPDX-FileContributor: Harald Eilertsen <haraldei@anduin.net>
  *
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
-
-function volse_init() {
-	App::$theme_info['extends'] = 'redbasic';
-}