Expose the Nix development environment

[mveril]

Summary

• share the development package list between the shell and a buildable profile
• expose packages.dev-environment for editors and non-interactive processes
• keep the standard scientific Python environment in that profile

Dependency

This PR is stacked on fix/nix-python-pyscf and should be retargeted to main after that fix merges.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features

Nix build validation will run on Linux CI.