feat: add flox envs command

[ysndr]

This PR adds an environment registry and flox envs command.

Flox commands has so far been mostly stateless, i.e. the fact that an environment exists at a given path is "forgotten" as soon as the command completes.
flox envs is supposed to list both active and available/inactive environments (that are represented by .flox directories).
To avoid scanning for .flox directories, we use the env registry introduced in #1312.

In addition, the flox envs supports filtering the output to only active environments as well as formatting its output as JSON.

[zmitchell]

Aside from the other comments I've left, it would be nice to write a couple of tests for get_registered_environments, etc.

stale