v4.0.0 Update WSK [Rollup.js]

• Update minimal requirement Node version > 10
• Replace Browserify with Rollup
• Remove gulp notify module
• Finally, we use Terser instead of gulp uglify
• Replace SCSS maps with CSS3 variables
• Add babelrc file into root folder
• Re-develop import variables. Now most of them are in gulpconfig file and pass them directly into tasks
• Add SCSS mixins for better performance

#5 #6