irc: contribute "light" theme overrides

Add irc-theme.{c,h} with the IRC plugin contribution to the built-in
"light" theme: 9 overrides on irc.color.* options tuned for a
light-background terminal (input_nick, item_lag_finished,
item_tls_version_deprecated, list_buffer_line_selected,
message_chghost, message_setname, nick_prefixes, topic_new, topic_old).

The registration is a small wrapper around weechat_theme_register that
builds a hashtable from a static (option, value) table and frees it
after the call. irc_theme_init() is called from weechat_plugin_init
after irc_config_init/read so the option names are already created
when the theme registry references them.

Default option values are NOT changed.

src/plugins/irc/irc-theme.h

@@ -0,0 +1,27 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef WEECHAT_PLUGIN_IRC_THEME_H
+#define WEECHAT_PLUGIN_IRC_THEME_H
+
+extern void irc_theme_init (void);
+
+#endif /* WEECHAT_PLUGIN_IRC_THEME_H */