edit prediction: Add syntax highlighting for diff popover (#23899)

Co-Authored-by: Antonio <antonio@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/copilot/src/copilot_completion_provider.rs

@@ -256,6 +256,7 @@ impl InlineCompletionProvider for CopilotCompletionProvider {
                 let position = cursor_position.bias_right(buffer);
                 Some(InlineCompletion {
                     edits: vec![(position..position, completion_text.into())],
+                    edit_preview: None,
                 })
             }
         } else {