Icon for .nu files' mimetype (#4813)

* Icon for .nu files' mimetype

Icon that should be displayed at every .nu script in a file explorer.

Later, a post-install script will be submitted; this script will install the mimetype+icon and the handler for doubleclicking/openning .nu files.

Signed-off-by: Daniell Mesquita <[email protected]>

* Complement previous commit

As per request, using author name in icon. Also, using the updated name.

Signed-off-by: Daniell Mesquita <[email protected]>