Move thread scopes into a separate header

The `thread_scope` enum is [gated behind the `atomic` header](https://github.com/NVIDIA/libcudacxx/blob/bda0c48d46ff7d0e3d9dea3240426efe56db6bc7/include/cuda/std/detail/__atomic#L65-L70).

The `atomic` header [emits errors when used with certain SM versions](https://github.com/NVIDIA/libcudacxx/blob/bda0c48d46ff7d0e3d9dea3240426efe56db6bc7/include/cuda/std/detail/__atomic#L9-L11).

`thread_scope` is useful outside of atomics for general scope labeling. It would be useful to have this enum in a separate header without the restrictions of ``. I'd like to see it treated as a `cuda::std` public API available on all platforms.

I'd also like to see it extended with a `warp` value for completeness, but I can make that a separate issue if it warrants more discussion.

cc: @wmaxey & @griwes who brought this up on slack. I've been using these enums in some prototypes and they're really useful.