Add Nushell support to venv activation (#3073)

This PR adds an option to run `activate.nu` in the automatic venv
activation code (relevant comment
[here](https://github.com/zed-industries/community/issues/2103#issuecomment-1742355651))

Release Notes:

- Added a `nushell` option to the
`terminal.detect_venv.on.activate_script` setting
([2103](https://github.com/zed-industries/community/issues/2103)).