Implement Runtime and Executor in iced_core

They can be leveraged by shells to easily execute commands
and track subscriptions.

core/src/lib.rs

@@ -44,3 +44,9 @@ pub mod subscription;
 
 #[cfg(feature = "subscription")]
 pub use subscription::Subscription;
+
+#[cfg(feature = "runtime")]
+mod runtime;
+
+#[cfg(feature = "runtime")]
+pub use runtime::Runtime;