Adds an icon for the block and also for the registered format type. Currently there is a generic "list view" icon and the format doesn't have any icon at all, which looks bad in the toolbar format menu:

With the icons, it looks like this:

What I did is that I extracted the "greek capital letter sigma" glyph from Times New Roman and exported it as SVG. Then rescaled it in Boxy to fit the "16x16 rectangle inside a 24x24 viewbox" convention, and that's all. I'm not sure if that's OK copyright-wise. Alternatively we could do the same thing with another font that is sufficiently public domain, like TeX's Computer Modern or Concrete Roman.