CEP idea: env name metadata

`conda` takes the directory name as the name of the environment too. This is used for CLI (`conda activate xyz`) and UX (the `(xyz)` prompt in the shell). This proposal would add a magic file under `conda-meta/` that proposes a name different that the environment directory.

Why is this needed or useful?

* conda-build temporary environments could be renamed underneath for better UX (e.g. the name of the package being built)
* pixi envs could be rendered as the project name instead of (.pixi/env).
* conda-store environments (which are named with hashes to implement atomic updates and rollbacks) could have a user-friendly name.
* Environment names could use spaces and other characters that are usually not very path friendly.

The idea would be very simple. If a `conda-meta/env-name` is present, parse the file contents and use that as the name.