Fix backspace on demo

buffer.x will probably be public API soon (#1994)

Fixes #1989

demo/client.ts

@@ -172,7 +172,7 @@ function runFakeTerminal(): void {
       term.prompt();
     } else if (ev.keyCode === 8) {
      // Do not delete the prompt
-      if (term.x > 2) {
+      if (term._core.buffer.x > 2) {
         term.write('\b \b');
       }
     } else if (printable) {