Use TerrainMaterials::from_str to parse material variant name

[kennethloeffler]

A FromStr implementation was added to TerrainMaterials in rbx_dom_weak 2.6.0. This PR removes the material_from_name function and changes Terrain:GetMaterialColor and Terrain:SetMaterialColor to use this new FromStr implementation to parse material variants from enum names. This reduces the amount of code in the terrain module and gives the responsibility of knowing all valid terrain materials to rbx-dom.

[filiptibell]

Thank you!