feat: venv backend fallback

[henryiii]

This allows a user to specify uv|virtualenv to use uv only if it is available. Also works for conda and mamba. A few notes:

• I used the | character for the way it looks in pipelines.
• A non-optional backend cannot be followed by an optional one; virtualenv|uv is invalid, for example.
• This is supported everywhere, but it's not really useful for the command line, so I didn't add it to the option system. It's most useful in a config file or maybe an environment variable.
• This also puts the environment selection into a dict, making it more reusable, and could allow power users (@wpk-nist-gov) to fiddle with adding new environment types. It is not considered supported/public, though.

I also made ProcessEnv a proper ABC, but technically could pull out that change, I think.

[theacodes]

Looks great.