Make timer method available on both foreground and background executors

Also, make it return a static future.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/util.rs

@@ -1,5 +1,6 @@
 use smol::future::FutureExt;
 use std::{future::Future, time::Duration};
+pub use util::*;
 
 pub fn post_inc(value: &mut usize) -> usize {
     let prev = *value;