Add a send button to the assistant (#12171)

![CleanShot 2024-05-22 at 18 20
45@2x](https://github.com/zed-industries/zed/assets/1789/dac9fcde-9fcb-4c40-b5da-ebdc847b3962)

Include the keybinding to help people discover how to submit from the
keyboard.

I'm also shifting from the word "Conversation" to "Context".

Release Notes:

- Added a send button to the assistant panel.

crates/assistant2/src/assistant2.rs

@@ -1108,7 +1108,7 @@ impl Render for AssistantChat {
                                     .on_click(cx.listener(move |this, _event, cx| {
                                         this.new_conversation(cx);
                                     }))
-                                    .tooltip(move |cx| Tooltip::text("New Conversation", cx)),
+                                    .tooltip(move |cx| Tooltip::text("New Context", cx)),
                             )
                             .child(
                                 IconButton::new("assistant-menu", IconName::Menu)