Improve keystroke search in keymap editor (#34567)

This PR improves Keystroke search by:

1.  Allow searching by modifiers without additional keys.
2. Take match count into consideration when deciding if we should show
an action as a search match.
3. Take order into consideration as well.

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>

crates/gpui/src/platform/keystroke.rs

@@ -417,6 +417,17 @@ impl Modifiers {
         self.control || self.alt || self.shift || self.platform || self.function
     }
 
+    /// Returns the XOR of two modifier sets
+    pub fn xor(&self, other: &Modifiers) -> Modifiers {
+        Modifiers {
+            control: self.control ^ other.control,
+            alt: self.alt ^ other.alt,
+            shift: self.shift ^ other.shift,
+            platform: self.platform ^ other.platform,
+            function: self.function ^ other.function,
+        }
+    }
+
     /// Whether the semantically 'secondary' modifier key is pressed.
     ///
     /// On macOS, this is the command key.