Function register_plugin_element 

pub fn register_plugin_element(
    registry: &dyn ThemeRegistry,
    plugin_id: &str,
    name: &str,
    doc: &str,
    spec: StyleSpec,
) -> String

The register-element host-service body. Registers into the SAME registry builtins use, owned by the plugin so unload can reverse it.

Returns the registered (namespaced) name so the caller can record a teardown token.