Recategorize Thermal Damage

[Turtlelord26]

Once we have anything in the rules that specifies high- or low-temperature thermal damage, we lose most of the benefit from combining Hot and Cold damage into Thermal since every source has to specify.

Solution: Reclassify Thermal damage as Thermal Damage, but list Heat Damage and Cold Damage as sub-types of Thermal damage.
Theory: This lets any effect that doesn't care say Thermal, but still allows ones that do care to say Heat or Cold. This is preferable to spelling out high-temperature Thermal damage over and over for word flow and comprehensibility reasons.