script: track per-script theme contributions and purge on script unload

Make individual scripts the unit of ownership for theme contributions
so that loading a script that calls weechat.theme_register(...) and
later unloading it correctly removes the script's overrides.

Plugin API addition (weechat-plugin.h):

  - new function pointer t_weechat_plugin.theme_unregister_script
    delegates to core's theme_unregister_script.
  - new convenience macro weechat_theme_unregister_script(script).
  - WEECHAT_PLUGIN_API_VERSION bumped to 20260704-03.

Script API additions (plugin-script-api.{c,h}):

  - new plugin_script_api_theme_register (plugin, script, name,
    overrides) forwards to the plugin API with the script pointer
    as the contribution owner, so contributions are tracked
    per-script (not per script-language plugin).

Lifecycle wiring (plugin-script.c):

  - new internal plugin_script_remove_themes (plugin, script)
    calls weechat_theme_unregister_script.
  - plugin_script_remove now calls it alongside the other
    plugin_script_remove_* helpers, so script-unload tears down
    everything (configs, bar items, themes, hooks).

Eight language bindings updated to call
plugin_script_api_theme_register instead of weechat_theme_register
directly, so they pass the script pointer as owner:

  - python, perl, ruby, lua, tcl, javascript, php, guile.

src/plugins/plugin.c

@@ -801,6 +801,7 @@ plugin_load (const char *filename, int init_plugin, int argc, char **argv)
         new_plugin->config_unset_plugin = &plugin_api_config_unset_plugin;
 
         new_plugin->theme_register = &theme_register;
+        new_plugin->theme_unregister_script = &theme_unregister_script;
 
         new_plugin->key_bind = &gui_key_bind_plugin;
         new_plugin->key_unbind = &gui_key_unbind_plugin;