Add 'Nebula Usage Score' curve input (#6926)

Background:
Many mods, especially Event Horizon, have highlighted how versatile and useful using nebula are for creating a wide variety of immersive effects beyond just nebula, including weather, volcanic ash, specialty debris falling from the sky, and more. In addition volumetric nebula have opened another realm of possibilities for modders to creatively utilize. Furthermore, modders can not leverage the powerful updates to the particle system to create a wide diversity of effects, which can be also utilized within nebula (or any mission) by making weapons with those effects and then spawning those weapons with scripts/sexps. Just one creative example is exploding rock debris from a volcano (like in Event Horizon). The visuals of nebula poof though are of course spawned in density according to the `Nebula Detail Level` graphics slider in the Graphics tab of the Options menu. Thus, what the player sets will affect visuals of the nebula. As a modder creating visuals, this can have the effect where particle effects spawned from weapons used in nebula may look different in less dense fields. Ideally there could be a way for the modder to incorporate this Nebula slider detail into the development of their effects, to ensure that the effect visuals properly reflect and integrate with the nebula density that the player might see. The solution to this problem is luckily not difficult, as PR #6889 shows how it can be dealt with. That PR added a particle curve input `Particle Usage Score` which is incredibly useful to allow particle effects to scale visually according to the `Particle Detail Level` that the player has set in the Graphics tab of the Options menu. As such this utility could also be extended to the Nebula detail level to allow modders to develop weapon spawned effects for nebula missions that properly visually integrate with the nebula detail level.

This PR
This PR follows and extends the relatively simple approach of #6889, in that it adds a new curve input `Nebula Usage Score` for use within the particle effects table. This new input will allow modders to develop effects that can properly mesh with varying nebula detail levels--all with the aim to further expand the visuals and range of nebula utility for environmental effects.

Author: wookieejedi

code/particle/ParticleEffect.h

@@ -234,6 +234,10 @@ class ParticleEffect {
 		std::pair {"Particle Usage Score", modular_curves_math_input<
 		    modular_curves_global_submember_input<get_particle_count>,
 			modular_curves_global_submember_input<Detail, &detail_levels::num_particles>,
+			ModularCurvesMathOperators::division>{}},
+		std::pair {"Nebula Usage Score", modular_curves_math_input<
+		    modular_curves_global_submember_input<get_particle_count>,
+			modular_curves_global_submember_input<Detail, &detail_levels::nebula_detail>,
 			ModularCurvesMathOperators::division>{}})
 	.derive_modular_curves_input_only_subset<size_t>( //Effect Number
 		std::pair {"Spawntime Left", modular_curves_functional_full_input<&ParticleSource::getEffectRemainingTime>{}},