Makes base artillery shells 2x cheaper and adds a new Explosive artillery shell that is more powerful but more expensive. Fork from https://mods.factorio.com/mod/explosive-artillery by zorkzorkus.

I changed a few things for balancing with RampantFixed mod. Damage numbers for Explosive shells are adjusted:
- Explosion area 12
- 2000 physical
- 2000 explosion
- 2000 electric
- 2000 fire