Require that vertex shaders return a position? #1567

MSL requires that if a vertex shader does not return void, then it must at least return the position builtin. We should consider whether WGSL should impose the same requirement, and also whether it should be valid for a vertex shader to return void.

I'd like to propose that WGSL should require vertex shader return types to include [[builtin(position)]] if they are not void. I do not have a strong opinion on whether or not we should allow a return type of void.