New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/assistant/src/inline_assistant.rs

@@ -1246,7 +1246,7 @@ impl InlineAssistant {
                     });
 
                     enum DeletedLines {}
-                    let mut editor = Editor::for_multibuffer(multi_buffer, None, true, window, cx);
+                    let mut editor = Editor::for_multibuffer(multi_buffer, None, window, cx);
                     editor.set_soft_wrap_mode(language::language_settings::SoftWrap::None, cx);
                     editor.set_show_wrap_guides(false, cx);
                     editor.set_show_gutter(false, cx);
@@ -1693,7 +1693,6 @@ impl PromptEditor {
                 },
                 prompt_buffer,
                 None,
-                false,
                 window,
                 cx,
             );