Implement Terrain:GetMaterialColor and Terrain:SetMaterialColor

[kennethloeffler]

Closes #90 by implementing the Terrain:GetMaterialColor and Terrain:SetMaterialColor methods.

I decided not to support passing strings to the material argument. This would require a bit more rigamarole to convert the string to an enum, and iirc Roblox recommends using enums over strings anyway. I'm open to adding it if complete parity with Roblox's implementation is important

I also decided to switch Lune's Color3 <-> Color3uint8 conversions so they just call rbx-dom's implementations, because Lune's From<Color3> for DomColor3uint8 was not quite correct, and they might as well use what's already there

[filiptibell]

A couple nitpicks with error messages / naming, other than that this looks great and is also really consistent with the rest of the codebase. Good catch with the color conversions too 👍

[filiptibell]

Thank you!

[hoontee]

Thanks!