Bundled CSS does not have source maps

[joews]

#99 moves SCSS compilation to rollup-plugin-sass.

It seems that options.sourceMap should make node-sass include source maps, but the file is not generated with:

plugins: [
    sass({
      output: 'dist/material-datetime-picker.css',
      options: {
       sourceMap: 'dist/material-datetime-picker.css.map'
      }
    }),

sourceMapEmbed does generate an inline source map, but it is somehow invalid; the browser does not interpret it:

plugins: [
    sass({
      output: 'dist/material-datetime-picker.css',
      options: {
       sourceMapEmbed: true
      }
    }),

I will merge #99 regardless, because it fixes so many other issues. Plus we have a single SCSS source file with little nesting, so source maps aren't really necessary.