Implement Default for CompactFormatter

[SOF3]

This allows libraries to provide an API like

impl<W, F> SomeWrapper<W, F> {
    pub fn new_serializer(writer: W) -> impl serde::Serializer
    where
        F: Default,
    {
        serde_json::Serializer::with_formatter(writer, F::default())
    }
}

[dtolnay]

Thanks!