Modernize and simplify CSS

[kytta]

This PR remakes the whole CSS used in the package.

• old position: absolute hacks get replaced with Flexbox
• instead of background-image, icons use mask-image. This allows us to change their colour based on button text colour
• the variables and calc()s aren't compiled away any more, but exposed for customizability. This makes the CSS a bit heavier, but it allows very easy customization
• instead of multiple PostCSS plugins, Lightning CSS is now used to minify styles. It's way faster and more effective

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!

[github-actions]

size-limit report 📦

Path	Size
./dist/shareon.min.css	5.44 KB (+0.38% 🔺)
./dist/shareon.es.js	879 B (0%)