Show appropriate activation command based on shell detection

[charliermarsh]

Summary

Closes #2174.

Test Plan

On Nushell:

(uv) ~/workspace/uv> cargo run venv                                                                                                                                                                                      
Using Python 3.12.0 interpreter at: /Users/crmarsh/workspace/uv/.venv/bin/python3
Creating virtualenv at: .venv
Activate with: overlay use .venv/bin/activate.nu

On Bash:

❯ cargo run venv "foo bar"
Using Python 3.12.0 interpreter at: /Users/crmarsh/.local/share/rtx/installs/python/3.12.0/bin/python3
Creating virtualenv at: foo bar
Activate with: source 'foo bar/bin/activate'