Simplify CSS

[kytta]

This PR remakes the whole CSS used in the package.

• [x] old position: absolute hacks get replaced with Flexbox
• [x] the variables and calc()s aren't compiled away any more, but exposed. This makes the CSS a bit heavier, but it allows very easy customization
• [x] instead of PostCSS, Lightning CSS is now used to minify styles. It's way faster and more effective, and it doesn't need plugins to operate.

Overall, the appearance of the buttons has become way more customizable. One can make the buttons vertical with a single line of CSS, change their radius and size, and more!

What else I want to do:

• [ ] see, if I can somehow tie Iconify with Lightning CSS to pull the icons on build and inline them in the final CSS. This might help a lot with #26

[github-actions[bot]]

size-limit report 📦

Path	Size
./dist/shareon.min.css	5.65 KB (+1.55% 🔺)
./dist/shareon.es.js	914 B (0%)

[kytta]

Surpassed by #74