No API to retrieve the schema type

While using the Rust crate for ada-url I have noticed that there's only `url.host_type()` but there's no getter for the schema type. It'd be great if we could expose the schema type as well so we have the same API surface as the C++ lib. For example: ```rust let url = Url::parse(href, None).expect("bad url"); let schema_type = url.schema_type(); ```