[CEP idea] Define what OS virtual packages should expose

[chenghlee]

As discussed on Gitter and the 2024-03-27 community meeting, we should write a short CEP that defines what information the OS virtual packages (__linux, __osx, __win, etc.) should provide (e.g., what values go into the package name, version, and build). We decided a CEP was needed because there are multiple conda-compatible implementations (e.g., conda, mamba, pixi), and a CEP would allow these implementations to coordinate.