Show appropriate activation command based on shell detection (#2221)
## Summary
Closes https://github.com/astral-sh/uv/issues/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'
```
