Add `MultiEnv`

Should run in parallel:
```py
import subenv

# Paths to Python interpreters (e.g. different conda, uv, or venv environments)
OLD = "/old/starsim/env/bin/python"
NEW = "/new/starsim/env/bin/python"

menv = subenv.MultiEnv([OLD, NEW])

menv('import starsim as ss')
menv('res = ss.Sim().run().results')
menv.compare('res') # or menv['res'].compare()
```