Add support for resizing splits and docks, as well as utilities (#3595)

Making this PR to upstream changes to util and GPUI2, resizing exists
but isn't working yet.

Release Notes:

- N/A

crates/gpui2/src/platform/test/platform.rs

@@ -12,6 +12,7 @@ use std::{
     path::PathBuf,
     rc::{Rc, Weak},
     sync::Arc,
+    time::Duration,
 };
 
 pub struct TestPlatform {
@@ -274,4 +275,8 @@ impl Platform for TestPlatform {
     fn delete_credentials(&self, _url: &str) -> Result<()> {
         Ok(())
     }
+
+    fn double_click_interval(&self) -> std::time::Duration {
+        Duration::from_millis(500)
+    }
 }