Use Instant from iced_core instead of std

This is needed for Wasm compatibility.

widget/src/shader/event.rs

@@ -1,10 +1,9 @@
 //! Handle events of a custom shader widget.
 use crate::core::keyboard;
 use crate::core::mouse;
+use crate::core::time::Instant;
 use crate::core::touch;
 
-use std::time::Instant;
-
 pub use crate::core::event::Status;
 
 /// A [`Shader`] event.