Provide an environment variable to automatically create or require a virtual environment

[brettcannon]

Like how Hatch creates an environment automatically, having a way to signal to the Launcher that an environment should always be used by a project might be helpful.

The way I currently see it in my head is:

1. Provide some enviroment variable (e.g. PY_CREATE_VIRTUALENV) that when set means the Launcher will create a virtual environment if one isn't found.
2. If PY_CREATE_VIRTUALENV is anything but the empty string or 1, it's used as the command to pass to the selected Python interpreter to create the virtual environment (not sure if string substitution like {version} when a version is specified/found would be useful).
3. An environment is only created if a pyproject.toml is found in the current or parent folders to act as an anchor point for the workspace.
4. Probably should have some output to stdout with a spinner indicating that the environment is being created.

I'm not sure if automatic installation into that environment makes sense.

It might also make more sense to not have this built in but instead to have a py pip subcommand (see #222 ) which will do the creation as necessary. Instead py could have a PY_REQUIRE_VIRTUALENV instead which will refuse to run if a pyproject.toml is found but no virtual environment (similar to what PIP_REQUIRE_VIRTUALENV does).