Allow an initial prompt to be associated with inline assist (#14816)

Release Notes:

- Added the ability to create custom inline assist bindings that
pre-fill a prompt from your keymap, e.g.:
    ```json
    [
        {
            "context": "Editor && mode == full",
            "bindings": {
                "ctrl-shift-enter": [
                    "assistant::InlineAssist",
                    { "prompt": "Build a snake game" }
                ]
            }
        }
    ]
    ```

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/quick_action_bar/src/quick_action_bar.rs

@@ -142,14 +142,14 @@ impl Render for QuickActionBar {
             "toggle inline assistant",
             IconName::MagicWand,
             false,
-            Box::new(InlineAssist),
+            Box::new(InlineAssist::default()),
             "Inline Assist",
             {
                 let workspace = self.workspace.clone();
                 move |_, cx| {
                     if let Some(workspace) = workspace.upgrade() {
                         workspace.update(cx, |workspace, cx| {
-                            AssistantPanel::inline_assist(workspace, &InlineAssist, cx);
+                            AssistantPanel::inline_assist(workspace, &InlineAssist::default(), cx);
                         });
                     }
                 }