Provide maskable icon

[hupf]

According to Lighthouse and this Google Developers resource, at least one maskable icon should be provided.

Maskable icons are a new icon format that give you more control and let your Progressive Web App use adaptive icons. If you supply a maskable icon, your icon can fill up the entire shape and look great on all Android devices. Firefox and Chrome have recently added support for this new format, and you can adopt it in your apps.

The Styleguide should provide such an icon that respects the minimum safe zone according to the [standard(https://w3c.github.io/manifest/#icon-masks).