feat: scroll-to-bottom on output via renderer pin tracking

Co-Authored-By: Sachin <sachinbeniwal0101@gmail.com>

src/apprt/surface.zig

@@ -108,6 +108,10 @@ pub const Message = union(enum) {
     /// Selected search index change
     search_selected: ?usize,
 
+    /// Scroll the viewport to the bottom. This is triggered by the renderer
+    /// when new output is detected and scroll-to-bottom on output is enabled.
+    scroll_to_bottom,
+
     pub const ReportTitleStyle = enum {
         csi_21_t,