Add Custom Death Explosions for striders

[StuffPhotonDev]

Implement The following Death Explosions:

• [ ] Purifier: Probably make it's turrets fall off it, which becomes 3 separate units that can continue shooting. Also probably reduce the wreck values of each turret from the wreck values of the main purifier so that when trying to rez, you'll have to pay the cost of the each turret instead of getting free ones again when the purifier dies again.

These death explosion Ideas need finalising:

• [ ] Vulcan: maybe have it spray naplam and napalm missiles from it's launcher as it's dying?

Think of death explosions for the following striders:

• [ ] Nebula
• [ ] Detriment (prehaps simply giga-nuke would do?)