Fix next/prev shortcuts handling in the File Finder (#9785)

This PR fixes the unexpected File Finder behaviour described in
https://github.com/zed-industries/zed/pull/8782#issuecomment-2018551041

Any change of the modifier keys except for the release of the initial
modifier keys now prevents opening the selected file.

Release Notes:

- N/A

crates/gpui/src/platform/keystroke.rs

@@ -213,6 +213,14 @@ impl Modifiers {
         }
     }
 
+    /// helper method for Modifiers with just control
+    pub fn control() -> Modifiers {
+        Modifiers {
+            control: true,
+            ..Default::default()
+        }
+    }
+
     /// helper method for Modifiers with just shift
     pub fn shift() -> Modifiers {
         Modifiers {