Support venv

[lolpack]

The type checker and IDE should be aware of venvs that are configured by users.

[connernilsen]

How to get this working for now:

If you have a venv set up locally, you can get Pyrefly working with it by having your venv sourced
in your shell (source .venv/bin/activate), and we will automatically pick up your installed packages. To pick
up your packages even when your environment isn't sourced, you can add .venv/bin/python3 (or
<path_to_venv>/bin/python3) to your Pyrefly configuration under
python_interpreter or pass it in with the --python-interpreter flag.

NOTE: You'll likely want to remove this from your config once we get auto-venv working, since
it may make your configuration more difficult to use on different systems.

[connernilsen]

Work is in progress for this, hoping to have this done by Friday.

[connernilsen]

It's done! a80f3fd

[lolpack]