Take a mutable context when resolving selections (#19948)

This is a behavior-preserving change, but lays the groundwork for
expanding selections when the cursor lands inside of a "replace" block.

Release Notes:

- N/A

crates/markdown_preview/src/markdown_preview_view.rs

@@ -301,8 +301,8 @@ impl MarkdownPreviewView {
                     this.parse_markdown_from_active_editor(true, cx);
                 }
                 EditorEvent::SelectionsChanged { .. } => {
-                    let editor = editor.read(cx);
-                    let selection_range = editor.selections.last::<usize>(cx).range();
+                    let selection_range =
+                        editor.update(cx, |editor, cx| editor.selections.last::<usize>(cx).range());
                     this.selected_block = this.get_block_index_under_cursor(selection_range);
                     this.list_state.scroll_to_reveal_item(this.selected_block);
                     cx.notify();