Make DisposalThread reachable (#235)

The `Config` object provided to `Call`s and `Driver`s allows setting a `DisposalThread`, but since it is unreachable from outside the crate, the only way to properly set it is using a `Songbird` instance, which may not be adequate for all use cases. This PR just makes `DisposalThread` reachable from the outside.

src/driver/mod.rs

@@ -64,6 +64,8 @@ use core::{
 use flume::{r#async::RecvFut, SendError, Sender};
 #[cfg(feature = "builtin-queue")]
 use std::time::Duration;
+#[allow(unused_imports)]
+pub use tasks::disposal::DisposalThread;
 use tasks::message::CoreMessage;
 use tracing::instrument;