assistant2: Add create buffer tool (#11219)

This PR adds a new tool to the `assistant2` crate that allows the
assistant to create a new buffer with some content.

Release Notes:

- N/A

---------

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

crates/gpui/src/view.rs

@@ -1,3 +1,4 @@
+use crate::Empty;
 use crate::{
     seal::Sealed, AnyElement, AnyModel, AnyWeakModel, AppContext, Bounds, ContentMask, Element,
     ElementId, Entity, EntityId, Flatten, FocusHandle, FocusableView, GlobalElementId, IntoElement,
@@ -457,3 +458,12 @@ mod any_view {
         view.update(cx, |view, cx| view.render(cx).into_any_element())
     }
 }
+
+/// A view that renders nothing
+pub struct EmptyView;
+
+impl Render for EmptyView {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        Empty
+    }
+}