Add icon component

[Syndesi]

Currently code regarding icons is split and duplicated across the different frontend repositories. As this setup is not maintainable, this repository should combine and focus icon components and related logic & styles.

The following things should be done:

• Icon CSS variables.
• Icon CSS styles.
• A simple lit-template function capable of rendering an icon, provided via SVG text.
• Installing the package @mdi/svg within this project, and documenting it in the readme.
• New minor release.