Allow `return` or `return nothing` in kernels

[eschnett]

I use JuliaFormatter to format my source code. This adds explicit return statements. This leads to

ERROR: LoadError: Return statement not permitted in a kernel function sum2_kernel!

even if the return statement is just return nothing.

At the same time, ending a kernel with an expression (which is equivalent to a return statement) is allowed.

Could this check be changed to allow only empty return or return nothing statements?

[vchuravy]

At the same time, ending a kernel with an expression (which is equivalent to a return statement) is allowed.

KernelAbstractions will politly ignore those and insert a return nothing for you. But yes we could explicitly allow return or return nothing

[pxl-th]

Closed by #446.