Enhancements code for easier for unit test

[phuanh004]

• [x] Split code into small functions
  • [x] Separate UI render code, and calculation code for later unit test
  • [x] Convert to async/ await
  • [x] #14

[phuanh004]

Hard to test if using one js file, so I will split js into 2 parts. The code is now using a module, so it won't work with only .html drag-n-drop to the website anymore.