Add ready-to-use CSS to distribution package

[adamkudrna]

All we need is to document two styling options:

1. Import a ready-to-use compiled CSS reactui.css.
2. Import source Sass files from src/scss.

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All Sass endpoints should be compiled to CSS, minified (but keep the unminified version too), and placed in dist:

• foundation(.min).css
• helpers(.min).css
• theme(.min).css
• components(.min).css

src can then be excluded from publishing. Sass source files remain intact.

Suggested Webpack plugin (maybe it's no longer the right choice): https://webpack.js.org/plugins/mini-css-extract-plugin/#minimal-example

[hubacekj]

The mini-css-extract-plugin still seems as the right choice, but it needs Webpack 5 to work (#358).

[mbohal]

I will work on it along with #373