[FEA]: Expose compiler detection macros

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

libcu++

### Is your feature request related to a problem? Please describe.

Any mature downstream project will at some point care about which compiler is currently being invoked. Usually, people end up rolling their own compiler detection macros with varying levels of success.

### Describe the solution you'd like

Expose the `_CCCL_COMPILER(x)` macros publicly (sans underscore) to users in a `cuda/compiler_detection` (name subject to bike-shedding) header.

Effectively do https://github.com/NVIDIA/cccl/pull/3237 but as a public interface.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_