pub fn register_widget<W: Widget>()Expand description
Register W: the lumenc HTML parser accepts <tag ...> for its
Widget::parser_tag, and every element spawned for that tag is handed,
with its authored attributes, to Widget::spawn.
Called by Widget derive output (<Type>::register(), which the
generated plugin’s build calls) at app startup, before the parser runs.
Registering again is a no-op; a second widget registered for the same tag
replaces the first.