Progress bar not updating during code upload

[stweedo]

The progress bar shows but never fills when uploading code. The issue is in serial.js where the uart.writeProgress is defined as an empty function:

  var uart = {
    writeProgress : function() {} // Dummy implementation - actual progress tracked in writeSerialWorker
  };

Fix:

  var lastSent = 0;
  var uart = {
    writeProgress : (sent) => {
      if (!sent) lastSent = 0;
      else if (sent > lastSent) {
        Espruino.Core.Status.incrementProgress(sent - lastSent);
        lastSent = sent;
      }
    }
  };

This tracks the cumulative bytes sent and updates the progress bar with incremental progress.