More UV Channel options

[Hayden-Fluff]

I often find myself unable to use some textures I have with some models I'm working with due to the fact the shader only supports 2 UV channels.

Most other shaders I've worked with that support multiple UV channels (With the exception of Unity Standard) support up to at least 4 UV channels, with some supporting more (highest I've seen is up to 8)

I'm not sure if this would be in scope for the shader (or how much it'd impact performance, I'm not too familiar with shader architecture myself), though this would allow me to create the effects I desire as this Toon shader mostly fills my style needs.