Improve the checkbox component

[KelvinOm]

1. Add new token for the border --border-width-2 with 2px value and use it in checkbox component
2. Reduce the size(height and width) of the checkbox from 5x to to 4x of root unit.
3. Improve border radius. The checkbox should not look like a radio button.

[KelvinOm]

Hey team! Please add your planning poker estimate with Zenhub @ichik @jsartisan @KelvinOm