Add a new #GtkSourceCompletionProvider to the completion object. This will add a reference @provider, so make sure to unref your own copy when you no longer need it.
a #GtkSourceCompletionProvider.
%TRUE if @provider was successfully added, otherwise if @error is provided, it will be set with the error and %FALSE is returned.
GException on failure.
See Implementation
Add a new #GtkSourceCompletionProvider to the completion object. This will add a reference @provider, so make sure to unref your own copy when you no longer need it.