Added icons SVG for easy import to icomoon

This adds the json file exported by [IcoMoon](https://icomoon.io/) to the assets folder. It's unnecessary for the theme, but is useful if you want to add, edit, or remove icons from the set by importing it.

The docs currently say you can import the .svg file, which is true, but you lose all the meta information and have to rename the icons. This fixes that limitation.

Kudos to @tranhelen for the catch.