Splitting specular_color between dielectric tint and metallic edge color

Currently, the `specular_color` parameter serves two purposes: It acts as a multiplier on top of the Fresnel term for the dielectric reflection, and it acts as the F82 parameter for the metallic component.

I'm not sure if this is a good idea, since these two purposes seem quite different to me:
- The Fresnel multiplier is a non-physical tweak, while the metallic parameter models a physical effect
- The Fresnel multiplier affects all angles, while the metallic parameter only affects near-grazing angles
- Sharing the parameter makes it impossible to create a half-metallic material which matches a real metal, but doesn't tint the dielectric reflection (of course, this sort of half-metallic half-dielectric material is not really physical, but users will do it anyways)

No files, tests, or entry points are named. Start by resolving the design question in the issue: determine whether dielectric tint and metallic edge color should be separate parameters, and define the intended behavior and acceptance criteria for that split.

Written by the indexing model from the issue text.