Moved template directories into templates/ subdirectory (templates/vanilla/
and templates/nextjs/) with complete Nix configuration in each. This ensures
initialized projects have the full development environment with Nix, Bun,
and all tooling out of the box.
Changes:
- Created templates/vanilla/ with flake.nix, nix/, .envrc, and all files
- Created templates/nextjs/ with flake.nix, nix/, .envrc, and all files
- Updated template paths in nix/modules/template.nix
