Add ICOSA vendor prefix

[mikeskydev]

Hi, I'd like to request the ICOSA vendor prefix for the Icosa Foundation.
We're working on extensions for expanding dynamic material and background scene support inside our suite of FOSS applications, namely Open Brush. We're building on top of Google's Tilt Brush and it's GOOGLE_tilt_brush_material extension.

Contact info:

Icosa Foundation
https://icosa.foundation

Thanks!