editor: Show hints for using AI features on empty lines (#20824)

Co-Authored-by: Thorsten <thorsten@zed.dev>
Co-Authored-by: Antonio <antonio@zed.dev>

Screenshot:

![screenshot-2024-11-18-17 11
08@2x](https://github.com/user-attachments/assets/610fd7db-7476-4b9b-9465-a3d55df12340)

TODO:
- [x] docs

Release Notes:

- Added inline hints that guide users on how to invoke the inline
assistant and open the assistant panel. (These hints can be disabled by
setting `{"assistant": {"show_hints": false}}`.)

---------

Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/gpui/src/window.rs

@@ -3050,7 +3050,7 @@ impl<'a> WindowContext<'a> {
     }
 
     /// Represent this action as a key binding string, to display in the UI.
-    pub fn keystroke_text_for(&self, action: &dyn Action) -> String {
+    pub fn keystroke_text_for_action(&self, action: &dyn Action) -> String {
         self.bindings_for_action(action)
             .into_iter()
             .next()
@@ -3065,6 +3065,26 @@ impl<'a> WindowContext<'a> {
             .unwrap_or_else(|| action.name().to_string())
     }
 
+    /// Represent this action as a key binding string, to display in the UI.
+    pub fn keystroke_text_for_action_in(
+        &self,
+        action: &dyn Action,
+        focus_handle: &FocusHandle,
+    ) -> String {
+        self.bindings_for_action_in(action, focus_handle)
+            .into_iter()
+            .next()
+            .map(|binding| {
+                binding
+                    .keystrokes()
+                    .iter()
+                    .map(ToString::to_string)
+                    .collect::<Vec<_>>()
+                    .join(" ")
+            })
+            .unwrap_or_else(|| action.name().to_string())
+    }
+
     /// Dispatch a mouse or keyboard event on the window.
     #[profiling::function]
     pub fn dispatch_event(&mut self, event: PlatformInput) -> DispatchEventResult {