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/assistant/src/prompt_library.rs

@@ -628,7 +628,7 @@ impl PromptLibrary {
         self.picker.update(cx, |picker, cx| picker.focus(cx));
     }
 
-    pub fn inline_assist(&mut self, _: &InlineAssist, cx: &mut ViewContext<Self>) {
+    pub fn inline_assist(&mut self, action: &InlineAssist, cx: &mut ViewContext<Self>) {
         let Some(active_prompt_id) = self.active_prompt_id else {
             cx.propagate();
             return;
@@ -636,9 +636,10 @@ impl PromptLibrary {
 
         let prompt_editor = &self.prompt_editors[&active_prompt_id].body_editor;
         let provider = CompletionProvider::global(cx);
+        let initial_prompt = action.prompt.clone();
         if provider.is_authenticated() {
             InlineAssistant::update_global(cx, |assistant, cx| {
-                assistant.assist(&prompt_editor, None, None, cx)
+                assistant.assist(&prompt_editor, None, None, initial_prompt, cx)
             })
         } else {
             for window in cx.windows() {