ack processed data chunks

lib/terminal/terminal.dart

@@ -220,6 +220,7 @@ class Terminal with Observable implements TerminalUiInteraction {
   void write(String text) {
     _queue.addAll(text.runes);
     _processInput();
+    backend?.ackProcessed();
     refresh();
   }
 

lib/terminal/terminal_backend.dart

@@ -25,4 +25,7 @@ abstract class TerminalBackend {
 
   /// terminates this backend
   void terminate();
+
+  /// acknowledges processing of a data junk
+  void ackProcessed();
 }