Immediate edit step resolution (#16447)

## Todo

* [x] Parse and present new XML output
* [x] Resolve new edits to buffers and anchor ranges
* [x] Surface resolution errors
* [x] Steps fail to resolve because language hasn't loaded yet
* [x] Treat empty `<symbol>` tag as None
* [x] duplicate assists when editing steps
* [x] step footer blocks can appear *below* the following message header
block

## Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Peter <peter@zed.dev>
Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/editor/src/editor.rs

@@ -10388,6 +10388,10 @@ impl Editor {
         }
     }
 
+    pub fn default_fold_placeholder(&self, cx: &AppContext) -> FoldPlaceholder {
+        self.display_map.read(cx).fold_placeholder.clone()
+    }
+
     pub fn set_gutter_hovered(&mut self, hovered: bool, cx: &mut ViewContext<Self>) {
         if hovered != self.gutter_hovered {
             self.gutter_hovered = hovered;