Improve ns-install-cli

[jkulhanek]

This PR implements the following:

• All entrypoints are automatically registered for ns-install-cli completions
• ns-install-cli detects conda environment and instead of putting the activation scripts to bashrc or zshrc it uses the activation scripts of the conda environment.

[brentyi]

I need some more time to fully go through this but just wanted to say this looks really clean @jkulhanek!

Not in scope here, but I'm wondering if there's a clean way to merge some of this logic upstream into tyro, especially since the script looks even less nerfstudio-specific now. It'd be really cool if we had a completion setup flow like this for all tyro.cli() applications.