Accept static sizes as keyword arguments

Kernel closures support passing `ndrange` and `workgroupsize` as keyword arguments:
https://github.com/JuliaGPU/KernelAbstractions.jl/blob/97419620494baa2e45541a6f2015413d6fa9315b/src/KernelAbstractions.jl#L661

The kernel function itself probably should too, while it currently only accepts positional versions of these arguments (used to construct static sizes): https://github.com/JuliaGPU/KernelAbstractions.jl/blob/97419620494baa2e45541a6f2015413d6fa9315b/src/macros.jl#L42-L44

IMO this makes the code more readable.

The kernel argument handling is in src/macros.jl at lines 42-44; compare it with the keyword-argument support for kernel closures in src/KernelAbstractions.jl at line 661. Update the kernel interface so ndrange and workgroupsize can be passed as keyword arguments for static sizes, and verify that the keyword and positional forms behave consistently.

Written by the indexing model from the issue text.